Rapid analysis of the lysosomal system state in human peripheral blood leukocytes was carried out by means of flow cytometry of diluted blood supravitally stained with acridine orange. For each of the subjects studied (26 healthy donors, nine donors with a weak cold-like infection and 30 patients with severe wounds), all the leukocytes were distributed into two distinct cell peaks: one with activated (A) and one with inactivated (I) lysosomal apparatus. It is shown here that whereas the mean cell frequency ratio of the former to the latter peak (A/I) of healthy donors was 1.18 (SD = 0.35) and was consistent with a Gaussian distribution, this value was statistically significantly increased in patients with cold-like symptoms (mean = 2.72, SD = 0.59, P < 10(-5), forming a bimodal distribution when pooled with data for healthy donors, and it increased dramatically (up to 40 times) in patients suffering the aftermath of severe wounds. Therefore, this method, needing only a few drops of the patient's blood and taking 10-15 min, can serve as a marker of health status. In addition, we show that activation/inactivation of the lysosomal apparatus of human leukocytes in vivo proceeds in an 'all-or-none' fashion. It will be possible in the future to enhance the technique by means of evaluating the A/I cell frequency ratio for separate leukocyte types.