Toad splenic lymphocytes undergo blastogenic conversion in vitro in response to both phytohemagglutinin (PHA) and allogeneic splenocytes. At 26°C, maximum incorporation of tritiated thymidine occurs on day 3 in response to PHA, and on day 7 in a mixed lymphocyte culture (MLC). The response to PHA is concentration-dependent. The maximum response to PHA and to alloantigens varies from toad to toad, and is influenced by the type of serum used to supplement the culture medium. More lymphocytes cultured with xenogeneic serum and PHA undergo mitosis than those cultured with PHA and allogeneic serum. In contrast, allogeneic serum suppresses transformation in the MLC.