Nonylphenol, an estrogenic xenobiotic, was identified as a substance released from "modified" polystyrene centrifuge tubes. This compound, extracted and purified using methanol, flash chromatography, and gas chromatography-mass spectrometry, was found to induce cell proliferation and progesterone receptor expression in human breast cancer MCF-7 cells. It also triggered mitotic activity in rat endometrium, confirming the reliability of the MCF-7 cell proliferation bioassay. The estrogenic properties of nonylphenol and other alkylphenols suggest that their presence in plasticware used in experimental and diagnostic tests may lead to inaccurate results. These compounds, along with alkylphenol polyethoxylates, may also pose health risks to humans and the environment. Alkylphenols are used as antioxidants in the plastic industry and have been found to leach from plastics used in food processing and packaging. The study highlights the potential environmental and health hazards of alkylphenols, which can contaminate water and accumulate in organisms. Despite their relatively low estrogenic potency, chronic exposure may lead to toxic effects. The study also shows that alkylphenols can mimic estrogen activity in various biological systems, including the reproductive system, and may contribute to reproductive impairments. The research underscores the importance of using estrogen-sensitive cell models to assess the estrogenic activity of xenobiotics. It also emphasizes the need for further studies to evaluate the potential health risks of alkylphenols and their environmental impact. The findings suggest that the widespread use of plastics containing alkylphenols may introduce unknown sources of error in estrogen-related research and pose significant health risks to humans and animals.Nonylphenol, an estrogenic xenobiotic, was identified as a substance released from "modified" polystyrene centrifuge tubes. This compound, extracted and purified using methanol, flash chromatography, and gas chromatography-mass spectrometry, was found to induce cell proliferation and progesterone receptor expression in human breast cancer MCF-7 cells. It also triggered mitotic activity in rat endometrium, confirming the reliability of the MCF-7 cell proliferation bioassay. The estrogenic properties of nonylphenol and other alkylphenols suggest that their presence in plasticware used in experimental and diagnostic tests may lead to inaccurate results. These compounds, along with alkylphenol polyethoxylates, may also pose health risks to humans and the environment. Alkylphenols are used as antioxidants in the plastic industry and have been found to leach from plastics used in food processing and packaging. The study highlights the potential environmental and health hazards of alkylphenols, which can contaminate water and accumulate in organisms. Despite their relatively low estrogenic potency, chronic exposure may lead to toxic effects. The study also shows that alkylphenols can mimic estrogen activity in various biological systems, including the reproductive system, and may contribute to reproductive impairments. The research underscores the importance of using estrogen-sensitive cell models to assess the estrogenic activity of xenobiotics. It also emphasizes the need for further studies to evaluate the potential health risks of alkylphenols and their environmental impact. The findings suggest that the widespread use of plastics containing alkylphenols may introduce unknown sources of error in estrogen-related research and pose significant health risks to humans and animals.