Effect of M. tuberculosis infection was studied on the expression of intercellular adhesion m olocule- 1 (ICAM-1) and Mac-1 markers on m urine peritoneal macrophages. Intraperitoneal administration of M. tuberculosis resulted in a m arked increase in the proportion of Mac-1＋ cells whereas the proportion of ICAM-1＋ cells declined sharply 4 h post infection. Absolute numbers of Mac-1＋ and ICAM-1＋ cells however increased at all time points after the infection. Comparison of kinetics of changes observed in Mac-1＋ and ICAM-1＋ cell populations with differential leukocyte counts in peritoneal cells indicated that these alterations could be due to cellular influx, especially that of neutrophils, or up regulation of these markers on macrophages and other peritoneal cells. In adherent peritoneal macrophages infected in vitro with M. tuberculosis, proportion of Mac-1＋ and ICAM-1＋ cells increased markedly within 24 h of infection. Mean expression of these markers on per cell basis also increased significantly. Similar results were obtained by using RAW 264.7 mouse macrophage cell line, suggesting that the enhanced expression of Mac-1 and ICAM-1 markers was a direct effect of M. tuberculosis infection and not mediated by contaminating cell types present in adherent macrophage preparations. Mac- 1 and ICAM-1 expression was further studied on macrophages that had actually engulfed M. tuberculosis and compared with bystander macrophages without intracellular M. tuberculosis. For this purpose M. tuberculosis pre-stained with DilC18 fluorescent dye were used for infecting adherent peritoneal macrophages. Mac-1 and ICAM-1 expression on gated DilC18 positive and negative cell populations was analyzed. Our results indicate that the expression of Mac-1 and ICAM- 1 markers was significantly enhanced on all macrophages incubated with M. tuberculosis but was more pronounced on macrophages with internalized mycobacteria. Taken together, our results suggest that the expression of Mac-1 and ICAM-1 markers is significantly up regulated as a result of exposure and infection with M. tuberculosis. Since these m arkers play important role in the uptake of mycobacteria as well as in the process of antigen presentation by macrophages, their upregulation may be beneficial for generation of a protective immune response to M. tuberculosis.