Using mAb ICO-160, we have studied Fas (APO-1/CD95) antigen expression on blood lymphocytes from healthy women, pregnant women and patients with chronic adnexitis, uterin myoma, ovarian cyst, ovarian and uterin cancer. In peripheral blood from healthy women Fas antigen was detected on 23.42 +/- 2.9% of lymphocytes. The healthy donors were divided in two different subgroups with low and high Fas expression. Expression of Fas antigen on lymphocytes was elevated (high expression subgroup) in all the groups of investigated patients, excepting the ovarian cancer ones with Fas expression similar to that in healthy donors. Comparison of expression of other differentiation antigens between healthy donors and the above groups of patients elucidated in the patients a kind of pronounced imbalance of the immune status and activation of the immune system. Additionally, in patients with ovarian cancer the imbalance of the immune status was reflected as altered ratio between helper and suppressor cells (the ratio was 0.73 in contrast to that 1.08 in group of healthy donors). As follows from the summarized results of all the trials, Fas antigen expression correlated with expression of other activation antigens as CD71 and CD25 (r = 0.05 and r = 0.52, respectively). Consequently, Fas (APO-1/CD95) antigen may be considered as the activation antigen and its expression may be used for assessing the immune status.