Chronic idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder in which the abnormality in cellular immunity has remained only vaguely defined. Previously we have shown that patients with ITP in its active phase have abnormal T cell subsets. We then examined the phenotypes of T and B lymphocytes in an additional 28 patients with ITP and 32 age- and sex-matched normal controls and compared the lymphocytes' capacity to respond to polyclonal T, T cell-dependent B, and B cell mitogens. Blastogenesis to optimal (5.0 micrograms/mL) and suboptimal (0.5 microgram/mL) concentrations of the polyclonal T cell mitogens were markedly depressed in patients compared with normal controls (P less than .0005). Similarly, a severe depression in response was noted with the polyclonal T cell-dependent B cell mitogen (P less than .000001). No difference was seen, however, with the polyclonal B cell mitogen. The proportions of pan-T and T helper/inducer lymphocytes were significantly depressed (P less than .005 and P less than .000005 respectively), and the T suppressor/cytotoxic lymphocytes increased (P less than .02) in patients relative to controls. But there was no difference in the proportion of B lymphocytes or in their functional response. The abnormal cellular immunity appears to be due to a defect in the T lymphocyte population without involvement of the B lymphocytes.