The mixed leukocyte reaction (MLR) is an in vitro test commonly performed in a serum-containing medium (SCM), and used to study allorecognition and cellular immunity accompanied by cytokine release. We investigated the possibility of performing the MLR test in serum-free media (SFM) by comparing human leukocyte proliferation and cytokine release in MLRs performed in SFM and SCM. Of the four SFM tested, only Biotarget- was as effective as SCM in supporting leukocyte proliferation and IL-2 secretion. Both phenomena were observed only in allogeneic combinations. The levels of IL-1, IL-6, and TNFalpha in allogeneic MLR combinations in SFM were half those in SCM cultures; the kinetics of their release were the same. With the exception of IL-2, a high degree of spontaneous release of the other three cytokines analyzed was observed in responder cells, in irradiated stimulator cells, and in autologous combinations cultured in both SCM and SFM. It appears that unlike IL-2, the cytokines IL-1, IL-6, and TNFalpha are nonspecifically produced in MLR and cannot serve as sensitive indices of HLA disparity.