T lymphocyte subsets expressing either T cell receptor alpha/beta or gamma/delta were selected from human peripheral blood T cells and proliferative responses to molecular mass-fractionated mycobacterial lysates were determined. alpha/beta T cells primarily responded to fractions greater than 30 kDa whereas gamma/delta T cells preferentially reacted to fractions less than 3 kDa. Protease digestion abolished the stimulating activities for alpha/beta T cells, confirming that alpha/beta T cells respond to protein components. In contrast, components recognized by gamma/delta T cells proved resistant to protease digestion. In limiting dilution studies, frequencies of proliferating gamma/delta T cells remained virtually unaltered by protease treatment of stimulating lysates, while those of alpha/beta T cells became almost undetectable. Furthermore, only few gamma/delta T cells responded to the 65-kDa heat-shock protein. Our data indicate that, unlike alpha/beta T cells, gamma/delta T cells respond to mycobacterial components which are resistant to vigorous protease digestion.