Flow cytometric analysis was carried out on peripheral blood cells from patients with tuberculosis (TB) (n = 84) and with mycobacteriosis other than tuberculosis (MOTT) (n = 38). A whole blood-staining-hemolysis procedure was used for the preparation of samples being analyzed, and the cells were double-stained with various combinations of fluorescein isothiocyanate (FITC)- and phycoerythrin (PE)- labeled monoclonal antibodies. These procedures enabled us to obtain quite reproducible results. As patients of more than 70 years old showed apparently distinct T lymphocyte profiles compared with those less than 70 years of age, this investigation was carried out only on patients of less than 70 years old. 1) The proportion of total lymphocytes to total leukocytes was significantly low in TB- and MOTT- groups, when compared with that in the healthy control group, although the total peripheral leukocyte number was not significantly different from each other. Thus, absolute numbers of lymphocytes were decreased significantly in TB- and MOTT- patients. 2) The numbers of both T and B lymphocytes in peripheral blood decreased in patients of both groups, leaving the ratio of T/B relatively constant. 3) Both CD4+/CD8- and CD4-/CD8+ subsets of T lymphocytes decreased in TB- as well as MOTT- groups. However, the decrease in CD4+/CD8- subset was more manifest than that in CD4-/CD8+ subset. Among CD4+/CD8- subset the proportion of the Leu8+ subpopulation was slightly lower and among CD4-/CD8+ subset CD11b- subpopulation was slightly higher in both TB- and MOTT- groups than in healthy control group. 4) There was no significant difference in proportions of IL-2-receptor (p55 alpha chain) positive as well as HLA-DR positive T-lymphocytes between patient groups and healthy control group. 5) Both TB- and MOTT- groups were subdivided according to the extent of pulmonary lesion. Patients with the larger lesion showed remarkable decreases in the ratio of T lymphocytes to total peripheral leukocytes, the number of T lymphocytes, and the numbers of CD4+/CD8- and CD4-/CD8+ subsets, when compared with those with the smaller lesion. 6) Although the averages of absolute numbers of T lymphocytes, CD4+/CD8- and CD4-/CD8+ subsets were lower in patient groups than in the control group and the ratios of these to total lymphocyte counts and the ratio of CD4+ to CD8+ subsets were not significantly different between patient groups and control group, the distributions of each value of individual person were far broad in patient groups.(ABSTRACT TRUNCATED AT 400 WORDS)