Two different human T cell leukemias were compared, using antiidiotype-like murine monoclonal antibodies. In each case these antibodies immunoprecipitated disulfide-linked heterodimer molecules from their respective leukemic cells. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the two idiotype-bearing molecules a major difference in molecular weight was observed, which could be attributed to a similar difference in size of the heavily iodinated chain of either heterodimer. The lightly iodinated chains of both molecules co-migrated at 43 Kd, but appeared to have different isoelectric points on two-dimensional gel analysis. The possibility that these two different heterodimers correspond to different classes of the putative T cell receptor for antigen is discussed. Assays of proliferation of the leukemic cells using Sepharose-bound antiidiotype-like monoclonal antibody showed that one of the leukemic cell types proliferated readily in response to its antiidiotypic antibody. This proliferation was not associated with measurable production of IL-2 and appeared to be a direct effect of the antiidiotypic antibody, which may mimic antigen in its interaction with the T cell receptor for antigen. The other leukemic cell type did not respond to Sepharose-bound antiidiotypic antibody and was generally unresponsive to lymphokines and mitogens. It is possible that the two leukemic cell types represent different stages of T cell differentiation.