In vitro long term induction of human peripheral blood lymphocytes by sodium diethyldithiocarbamate, DTC (Imuthiol) enhanced the expression of OKT and HLA-DR phenotypic determinants. A population of T-B-DR+ cells was recruited from the null cell population. The increase in OKT+ cells, but not the enhanced HLA-DR expression appeared to be macrophage dependent. The surface markers of NK cells, monocytes or the pokeweed mitogen-induced IgM production were not modified. Collectively, the findings indicate that Imuthiol is not a B cell inducer, but is specifically active on the T-cell population in the way it induces the maturation of null cells into OKT+ DR+ lymphocytes.