The causative agent of tuberculosis, Mycobacterium tuberculosis, has two chaperonin (Cpn60) proteins and one cochaperonin (Cpn10) protein. We show here that cpn60.2 and cpn10, but not cpn60.1, are essential for cell survival. A mutant lacking Cpn60.1 was indistinguishable from the wild-type organism in plate and broth culture and within murine macrophages, although it showed increased sensitivity to high temperature (55 degrees C). However, infection of mice with the Deltacpn60.1 mutant revealed a major difference from the wild-type organism. In spite of having equal numbers of bacteria in infected sites, the Deltacpn60.1 mutant failed to produce granulomatous inflammation in either mice or guinea pigs. This was associated with reduced cytokine expression in infected animals and macrophages. Cell wall lipid acid composition was not altered in the mutant strain. Thus, it appears that Cpn60.1 is an important agent in the regulation of the cytokine-dependent granulomatous response in M. tuberculosis infection.