Cellular-mediated immunity was studied in autoimmune thrombocytopenic purpura (ATP) patients by investigating leukocyte migration inhibition (LMI) following the interaction of normal platelets with patients' lymphocytes. When normal platelets were incubated with leukocyte buffy coats of ATP patients, the migration index (MI) was significantly impaired compared to buffy coats from normal subjects, employing 4 different concentrations of platelets. At the highest platelet concentration (10(9)/ml), MI was 0.87 +/- 0.04 (SEM) for ATP lymphocytes compared to 1.05 +/- 0.05 (p less than 0.01) for normal lymphocytes. Nine of 21 patients had an MI less than 0.80, whereas all control subjects had MIs greater than 0.85. Similar results were obtained at 2 different platelet membrane concentrations. At 500 micrograms/ml, the MI for ATP lymphocytes was 0.74 +/- 0.04, compared to 0.98 +/- 0.08 (p less than 0.01) for normal lymphocytes (12 experiments). An inverse relationship was noted between platelet count and lymphokine production in ATP patients (r = 0.815, p less than 0.001, 10 experiments). Transfer factor from an ATP patient in remission converted an abnormal LMI response of 0.68 +/- 0.04 from a patient with severe thrombocytopenia to 0.84 +/- 0.07 (p less than 0.005, 8 experiments). Similar results were obtained with transfer factor from 2 other patients in remission. Transfer factor from a patient with severe thrombocytopenia converted a normal response of 1.04 +/- 0.05 of normal subjects to a lower response of 0.88 +/- 0.04 (p less than 0.03, 12 experiments). Thus, lymphocytes of ATP patients are primed to recognize and be perturbed by normal platelets, whereas normal lymphocytes are not. This indicates specificity of the antigen- lymphocyte reaction in ATP patients. Transfer factor is capable of modulating this response in vitro.