AbstractThe anti-tumour antibiotic Bleomycin, the experimental immunostimulatory tripeptide FK 565, and the immunosuppressive agent Cyclosporin A were examined for theirin vitroeffects on the release of the haemopoietic cytokine Granulocyte Macrophage Colony-Stimulating Factor (GM-CSF) from LPS stimulated BALB/c adherent peritoneal macrophages. FK 565 and Bleomycin produced an increase in GM-CSF release indicating that these agents are capable of stimulating peritoneal macrophage populations. Cyclosporin A was found to have no effect.