Spontaneous proliferation and structural and functional properties of the cell membranes were studied by measuring activity and latent activity of lactate dehydrogenase isoenzymes in peripheral blood leukocytes from patients with disadaptation at admission to the hospital and during therapy. The spectrum of lactate dehydrogenase isoenzymes in patients with disadaptation at admission did not differ from the corresponding values in the control group. Therapy was accompanied by a shift in isoenzyme spectrum of lactate dehydrogenase toward the prevalence of one isoform, which probably reflected metabolic changes in immunocytes under conditions of adaptive immune response to psychoemotional stress. Latent activity of lactate dehydrogenase isoenzymes in patients at admission was below the control. Latent enzyme activity returned to normal during therapy. The observed changes in latent enzyme activity were probably associated with recovery of structural and functional properties of leukocyte membranes.