We studied host immune parameters which might be related to the activity and the pathogenetic mechanism of chronic active hepatitis. The subjects consisted of 45 cases with hepatitis B virus surface antigen (HBsAg)-positive chronic active hepatitis (CAH), 44 HBsAg-negative CAH, 22 with inactive chronic hepatitis, and 45 cases of normal persons, hepatitis B virus (HBV) carriers, or the patients with acute myocardial infarction. The in vitro assay for the in vivo activated lymphocytes was performed by measuring spontaneous thymidine uptake (SLT) of lymphocytes isolated from peripheral blood. SLT was significantly (p < 0.001) elevated in cases with HBsAg-positive (1227 ± 806 cpm) and-negative CAH (1017 ± 559 cpm) compared to the patients with inactive chronic hepatitis (347 ± 79 cpm) and to the control group (320 ± 106 cpm). SLT values observed in 7 cases with active disease (group I and II), in which remission and relapsing phase could be assessable, were elevated from 648 ± 121 cpm in remission phase to 1548 ± 606 cpm one to two weeks before the appearance of biochemical evidence (SGPT) of relapse. This pattern of SLT elevation, however, was not observed in patients with inactive hepatitis. Neither the abnormal distribution of T-celi subsets nor the presence of conventional HBV markers were related to the elevated SLT value. Our findings may therefore indicate that SLT might be useful in assessment of the disease activity in patients with CAH.