The immune responses of healthy recipients of Mycobacterium bovis bacille Calmette-Guérin (BCG) vaccine, tuberculosis (TB) patients, and contacts of TB patients were examined to three major secretory proteins of Mycobacterium tuberculosis, MPB59, MPB64, and MPB70. MPB59 evoked a T cell response in 78% of BCG vaccines, 62% of TB patients, and 60% of contacts. MPB64 and MPB70 were recognized by < 15% of BCG vaccinees, half of TB patients, and three-quarters of contacts. TB and leprosy patients had antibody responses to MPB59, but few had antibodies to MPB64 or MPB70. Hybridization of mycobacterial DNA with specific gene probes demonstrated the absence of a gene for MBP64 in the vaccine strain of BCG, but the MPB70 gene was found in all virulent and vaccine BCG strains tested. Since MPB64 and MPB70 can induce delayed-type hypersensitivity reactions in infected animals, either of these proteins may have potential as skin test reagents for detecting infection with M. tuberculosis.