Alpha 1-proteinase inhibitor (alpha 1-PI) has been demonstrated to suppress mitogen-induced lymphocyte response in vitro. To evaluate the effect of intravenous application of human alpha 1-PI (Prolastin HS) on cellular immunity, we determined total lymphocyte count, lymphocyte subsets and lymphocyte response to concanavalin A, before and 24 h after infusion of 60 mg.kg-1 body weight alpha 1-PI in eight patients with homozygous alpha 1-PI deficiency (PiZ phenotype). The results were compared with two blood samples from seven healthy controls. After infusion, serum alpha 1-PI levels were increased from 0.98 +/- 0.24 to 2.68 +/- 0.51 g.l-1. No significant differences were found for total lymphocyte count, lymphocyte subsets and lymphocyte response between both groups in both samples. Maximum 3H-thymidine incorporation before and after infusion showed no significant difference; the same was true for the two control samples. However, additional incubation in vitro with alpha 1-PI 5 g.l-1 led to a significant (p < 0.03) decrease of lymphocyte proliferation in samples after infusion. Our data indicate that alpha 1-PI substitution therapy does not lead to a major suppression of lymphocyte response to concanavalin A in PiZ individuals in vivo, although a suppressive effect was found after additional in vitro incubation with alpha 1-PI.