Monocytes and lymphocytes were separated from human peripheral blood, and their capacity to synthesize metallothioneins (MTs) was examined. Both cell types can produce proteins electrophoretically similar to HeLa cell MTs. These proteins are inducible by Cd2+ and specifically precipitable by anti-MT serum. To have an insight into the capacity of T- and B-lymphocytes to produce MTs, sensitivity to Cd2+ of both cell types was estimated during mitogen-specific blastogenesis. Lymphocytes were prepared from mouse spleen and stimulated by concanavalin A (ConA) or lipopolysaccharide (LPS) in the presence of Cd2+. [3H]thymidine incorporation, as well as [3H]uridine incorporation in an earlier stage, was more severely inhibited by Cd2+ in ConA-induced mitogenesis than in LPS-induced mitogenesis, indicating T-lymphocytes are more sensitive to Cd2+ toxicity. This may reflect lower capacity of T-lymphocytes to synthesize MTs in response to Cd2+.