1. Low mol. wt proteins with high Cd-binding capacity were found to be induced by Zn in cultured monocytes and lymphocytes. 2. In T cell cultures one protein was found to be induced by 125 microM Zn for 6 days, while in monocytes and B enriched cells under these conditions two proteins were found, of which the one with higher mol. wt had similar electrophoretic mobility to the T cell protein on polyacrylamide gels. 3. Mol. wt criteria and crossreactivity towards anti-metallothionein (Mt) antibody identified these proteins to be Mts of about 23 and 27 kDa mol. wt. 4. Metal binding studies indicated that monocyte and lymphocyte Mts had a higher affinity to Zn compared to Cd than rat liver Mt and Mts from Cd-resistant substrains of a human epithelial and a murine fibroblast cell line. 5. The presence of the T cell mitogen phytohemagglutinin (PHA) was found not to be necessary for this Mt induction by Zn.