Peripheral blood lymphocytes from untreated patients with essential cryoglobulinaemia were studied for their surface markers and for their in vitro mitogenic reactivity. No differences in lymphocyte subpopulations were observed between cryoglobulinaemic patients and normal controls. Cultures of separated lymphocytes were stimulated with different concentrations of phytohaemagglutinin, Con-A and pokeweed mitogen. Incorporation of [3H]-thymidine in patients' cultures was compared with that of normal controls. Significantly decreased reactivity to phytohaemagglutinin and Con-A, but not to pokeweed mitogen, was found in all patients studied. The depressed mitogenic reactivity to phytohaemagglutinin and Con-A might be referred to a qualitative T cell defect.