The in vitro proliferation of the spontaneous lymphoid T-cell leukemia designated LB was enhanced by physiological, intermediate and supraphysiological concentrations of insulin. The enhancing effect was observed in both serum-free medium (SFM) and medium containing low concentrations of serum. Guinea-pig anti-insulin serum, but not guinea-pig normal serum, inhibited the proliferation of LB cells incubated either in medium containing serum alone or in medium containing serum and supplemented with insulin. This finding suggests that LB cells use serum insulin as a growth factor. Insulin-like growth factors I (IGF-I) and II (IGF-II) failed to stimulate an appreciable proliferation in LB cells, whereas in the same experiment insulin markedly enhanced the proliferation of this lymphoid leukemia. Furthermore, the concentration of unlabelled insulin required to displace 50% of '251-insulin bound to LB cells was 3 orders of magnitude lower than the concentration of IGF-I required to achieve the same dispfacement. Our findings indicate that interaction of insulin with its own receptor, and not with IGF-I receptor, triggers the proliferation of LB cells. Radio-receptor assays revealed that LB cells ex ress approximately 3,200 molecules of high affinity (Kd = 10-M) insulin receptor per cell. None of 7 other tumor cell lines tested responded to insulin. The proliferation of insulin-stimulated LB cells was also inhibited with tyrphostin, a tyrosine kinase blocker analogous to tyrosine, which perhaps blocks the tyrosine kinase activity of the insulin receptor &chain.