In 1973, three breast tumor cell lines—MDA-MB-134, MDA-MB-175, and MDA-MB-231—were isolated from pleural effusions of breast cancer patients. These lines were characterized by distinct chromosome numbers (43, 49, and 65–69, respectively). Pleural effusions provided a rich source of viable tumor cells, with minimal contamination by fibroblasts, and allowed for sequential sampling from the same patient. The cells could be separated based on their attachment properties to culture flasks. All lines from different patients were distinct, while sequential samples from the same patient were similar. No viruses or mycoplasmas were detected in any line. Establishing tumor cell lines from solid breast carcinomas has been challenging. Pleural effusions, however, proved more successful due to their high viability, minimal fibroblast contamination, and ease of separation. The three cell lines described were derived from patients with breast cancer, each showing distinct characteristics. MDA-MB-134 had a mean chromosome number of 43, MDA-MB-175 had 49, and MDA-MB-231 had 65–69. These lines were established from pleural effusions and showed consistent growth in culture. The cells were studied using electron microscopy and histological techniques, confirming their epithelial origin. Chromosome analysis revealed hypodiploid, diploid, and near-triploid configurations, respectively. The study highlights the potential of pleural effusions as a reliable source for isolating breast tumor cells. The cell lines were cultured successfully, with MDA-MB-134 and MDA-MB-175 showing distinct growth patterns and chromosome numbers. MDA-MB-231 exhibited rapid growth and was able to form tumor nodules in nude mice. The cell lines were characterized by their ability to grow in culture, their distinct morphology, and their consistent chromosome numbers. The study also emphasizes the importance of sequential sampling and the use of appropriate culture media to maintain cell viability and purity. The results suggest that pleural effusions are a valuable source for studying breast tumor cells, with the potential for further research into their biological properties and therapeutic applications.In 1973, three breast tumor cell lines—MDA-MB-134, MDA-MB-175, and MDA-MB-231—were isolated from pleural effusions of breast cancer patients. These lines were characterized by distinct chromosome numbers (43, 49, and 65–69, respectively). Pleural effusions provided a rich source of viable tumor cells, with minimal contamination by fibroblasts, and allowed for sequential sampling from the same patient. The cells could be separated based on their attachment properties to culture flasks. All lines from different patients were distinct, while sequential samples from the same patient were similar. No viruses or mycoplasmas were detected in any line. Establishing tumor cell lines from solid breast carcinomas has been challenging. Pleural effusions, however, proved more successful due to their high viability, minimal fibroblast contamination, and ease of separation. The three cell lines described were derived from patients with breast cancer, each showing distinct characteristics. MDA-MB-134 had a mean chromosome number of 43, MDA-MB-175 had 49, and MDA-MB-231 had 65–69. These lines were established from pleural effusions and showed consistent growth in culture. The cells were studied using electron microscopy and histological techniques, confirming their epithelial origin. Chromosome analysis revealed hypodiploid, diploid, and near-triploid configurations, respectively. The study highlights the potential of pleural effusions as a reliable source for isolating breast tumor cells. The cell lines were cultured successfully, with MDA-MB-134 and MDA-MB-175 showing distinct growth patterns and chromosome numbers. MDA-MB-231 exhibited rapid growth and was able to form tumor nodules in nude mice. The cell lines were characterized by their ability to grow in culture, their distinct morphology, and their consistent chromosome numbers. The study also emphasizes the importance of sequential sampling and the use of appropriate culture media to maintain cell viability and purity. The results suggest that pleural effusions are a valuable source for studying breast tumor cells, with the potential for further research into their biological properties and therapeutic applications.