A method is described for assessing the chemotactic activity of soluble substances on motile cells. The technique involves a perspex chamber with a filter membrane that separates two compartments. Leucocytes are allowed to settle on one side, while a test solution is placed on the other. After incubation, the filter membrane is examined microscopically to count the number of cells that have migrated through. Antibody-antigen mixtures, when incubated at 37°C in fresh normal rabbit serum, exert a strong chemotactic effect on rabbit polymorphonuclear leucocytes. Results indicate that when antibody-antigen complexes are incubated in fresh serum, a heat-stable substance is produced, which acts directly as a chemotactic stimulus. This heat-stable substance is not produced when antibody-antigen complexes are incubated in serum heated at 56°C for 30 minutes. The study shows that the chemotactic effect of antibody-antigen mixtures is not due to the complex itself, but rather to a by-product of the interaction. The chemotactic substance is heat-stable and is produced when antibody and antigen interact in fresh serum. The presence of specific antiserum enhances the chemotactic effect of antigens like human serum albumin and ovalbumin, which are otherwise inactive in normal serum. The findings suggest that the chemotactic response involves a heat-labile component, possibly an enzyme, which interacts with the antibody-antigen complex to produce a heat-stable chemotactic substance. This substance directly influences the leucocytes, leading to directional migration. The results also indicate that the chemotactic effect may not be solely dependent on serum factors, as some bacterial products are more chemotactic than mammalian serum albumins. The study highlights the importance of understanding the mechanisms behind chemotaxis, particularly in the context of immune responses. The developed method provides a reliable way to assess chemotactic activity, which can be applied to other cell types, including lymphocytes. The findings have implications for understanding immune responses and may contribute to the development of treatments for inflammatory diseases.A method is described for assessing the chemotactic activity of soluble substances on motile cells. The technique involves a perspex chamber with a filter membrane that separates two compartments. Leucocytes are allowed to settle on one side, while a test solution is placed on the other. After incubation, the filter membrane is examined microscopically to count the number of cells that have migrated through. Antibody-antigen mixtures, when incubated at 37°C in fresh normal rabbit serum, exert a strong chemotactic effect on rabbit polymorphonuclear leucocytes. Results indicate that when antibody-antigen complexes are incubated in fresh serum, a heat-stable substance is produced, which acts directly as a chemotactic stimulus. This heat-stable substance is not produced when antibody-antigen complexes are incubated in serum heated at 56°C for 30 minutes. The study shows that the chemotactic effect of antibody-antigen mixtures is not due to the complex itself, but rather to a by-product of the interaction. The chemotactic substance is heat-stable and is produced when antibody and antigen interact in fresh serum. The presence of specific antiserum enhances the chemotactic effect of antigens like human serum albumin and ovalbumin, which are otherwise inactive in normal serum. The findings suggest that the chemotactic response involves a heat-labile component, possibly an enzyme, which interacts with the antibody-antigen complex to produce a heat-stable chemotactic substance. This substance directly influences the leucocytes, leading to directional migration. The results also indicate that the chemotactic effect may not be solely dependent on serum factors, as some bacterial products are more chemotactic than mammalian serum albumins. The study highlights the importance of understanding the mechanisms behind chemotaxis, particularly in the context of immune responses. The developed method provides a reliable way to assess chemotactic activity, which can be applied to other cell types, including lymphocytes. The findings have implications for understanding immune responses and may contribute to the development of treatments for inflammatory diseases.