Cancer patients are often treated with biological response modifiers to enhance immunological functions. However, little is known about the actual mechanism of action of many of these substances. Therefore, we were interested in the effect of i.p. treatment with porcine low-molecular-weight spleen peptides, which are used during supportive cancer therapy, on lymphoid cell populations and function in mice. After treatment with 0.5 microgram peptides/kg body weight for 14 consecutive days, lymphokine secretion and the generation of cytotoxic T-cells were significantly enhanced as compared with controls. However, there was no effect on the number of cells or the percentage of cells expressing functional surface markers in secondary lymphoid organs.