Summary:We examined the effect of levamisole (LMS) on the proliferative response and interleukin‐2 (IL‐2) concentration in OKT3‐, phytohemagglutinin‐, and concanavalin‐A‐stimulated lymphocyte cultures. Although proliferative response was enhanced in lymphocyte cultures. stimulated in the presence of LMS, similar levels of IL‐2 were observed in stimulated and unstimulated cultures. The mechanism of the enhancement effect of LMS on proliferative response was further characterized by studying its effects on the growth of IL‐2‐dependent CTLL‐2 cells in culture. Since this cell line has been shown to require 2‐mercaptoethanol (2‐ME) for normal growth in recombinant IL‐2, the effect of LMS on several parameters of its growth was compared with that of 2‐ME. Unlike 2‐ME, LMS did not enhance35S‐cystine uptake. Both compounds increased thiol concentration in the cell culture, but (oxidized) 2‐ME induced a greater increase. Generally, the effects of LMS on CTLL‐2 growth were quite similar to those of structurally unrelated compounds known to have antioxidant properties, and the demonstrated thiol requirement of this cell line for growth in recombinant IL‐2 was met by substituting LMS for 2‐ME. When the effect of LMS on IL‐2 receptor (IL‐2R) expression in CTLL‐2 cells was examined by a receptor‐ligand binding assay involving low levels (10‐80 pM) of125IL‐2, a modest increase in the level of IL‐2R expression was observed. The biologically active high‐affinity IL‐2R complex is believed to be preferentially bound at the low levels of125IL‐2 used here, suggesting a functional relevance for this effect of LMS. These observations should be useful in minimizing the cost and duration of in vitro expansion of lymphocytes for use in adoptive immunotherapy and should be applicable in improving the response of immunologically impaired patients to immunotherapy.