ABSTRACT— A variety of immunological parameters has been serially examined in the blood of 12 patients with acute post-transfusion hepatitis non-A non-B (PTHNANB). Several alterations of these tests were transiently detected when comparing with healthy, extensively transfused subjects as well as with normal volunteers: 1. Low proportions of CD8+ T lymphocytes were observed at the disease onset, and these returned to normal after 1 month; 2. A diminution of the proliferative response of blood lymphocytes to mitogens was detected during the same period of time; 3. By the third month of disease, an enhanced spontaneous IgG secretion by cultured lymphocytes was found, and this observation was restricted to those patients who had not recovered. These alterations suggest that the immune system might be involved in the course of hepatitis C virus infection.