AbstractBlastogenic transformation of murine spleen cells elicited with concanavalin A was suppressed by serotonin 10−-12to 10−-6M, and marginally stimulated by its antagonists ketanserin and propranolol in low concentrations (10−-15to 10−-11M). Ketanserin (5-HT2receptor ligand) and propranolol (5-HT1Aand beta-adrenergic ligand) did not block the suppressive effect of serotonin if used along with it in equimolar concentrations (10−-9M). Ergot-alkaloid dihydroergosine suppressed, whereas dihydroergotoxin stimulated the bldstogenic transformation. Opposite effects of the agents were obtained in experiments with mouse myeloma X63/Ag 8.653 and hybridoma SHV 125 cell lines, which unlike normal lymphoid cells, are homologous cell populations and proliferate spontaneously. The data indicate that serotonin and its antagonists interfere directly with mitosis and/or autocrine stimulation of target cells.