The aim of the present study was to better characterize the expansion of double-negative (DN) T cells in vivo in AIDS patients and to ascertain the discrepant response of an immunodepressed immune system towards two distinct mycobacterial infections. In a large cohort of HIV-1 seropositive patients with low CD4(+) T cell counts (<100/mm(3)), we have recently reported on an expansion of DN T cells which was observed only in patients with disseminated Mycobacterium avium infection, toxoplasmosis and Kaposi sarcoma, but not in patients with tuberculosis. The potential differential gammadelta T cells response observed in vivo in AIDS patients with tuberculosis or disseminated M. avium complex infection was investigated by collecting the concomitant or the closest T lymphocyte counts performed within 2 weeks of bacterial diagnosis of 112 disseminated M. avium infection and 41 tuberculosis patients. The DN and gammadelta T cell percentages were different between the two groups (P < 10(-4)) and the expansion of this compartment was found only with disseminated M. avium infections. An analysis of the variable delta2 segment versus pan-delta bearing T cells ratio disclosed a predominance of non-V(delta)2 T cells in these patients whose average values were identical in both groups. It is therefore concluded that the difference seen between these two types of mycobacterial infections concerning the DN T cells only involved the gammadelta T cells although the mechanism of their preferential expansion in disseminated M. avium infections remains a matter of speculation.