Strains H37Ra and H37Rv, attenuated and virulent variants, respectively, of the original human strain H37 of Mycobacterium tuberculosis, were used to infect cultures of mouse peritoneal macrophages. Bacterial viability of each strain was assessed over a 2-week period, and the cellular response to H37Ra during the first week was observed using electron microscopy. Prelabeling of secondary lysosomes with ferritin was used to facilitate the estimation of fusion of the lysosomes with phagosomes containing the bacteria. Streptomycin was excluded from the medium of cell cultures infected with H37Ra. The intracellular viability of strain H37Rv (in the presence of streptomycin) showed a lag during the first week after infection and then rose progressively to a mean figure seven times the starting level. In contrast, the viability of strain H37Ra declined, on the average, to one-fifth of the starting level during the first week; moreover, this decline occurred in the absence of antibiotics. In the second week a variable rise in the viable count took place, usually regaining the starting level. Electron microscopy of macrophages infected with H37Ra revealed a higher proportion of "damaged" bacteria 5 days after infection than at 1 day, in keeping with the decline in viability. Phagosomes containing these "damaged" (and presumed dead) organisms showed virtually universal fusion with prelabeled lysosomes. Phagosomes containing "intact" bacteria of this strain showed a prevalence of fusion varying from 38 to 56%, somewhat higher than the level previously reported for "intact" organisms of H37Rv. Nevertheless, the lysosome-phagosome fusion response to "intact" H37Ra was still far less extensive than that observed previously towards "intact" M. lepraemurium (around 90%). In conclusion, a difference between the macrophage lysosome-phagosome fusion response towards viable organisms of strain H37Ra and to the virulent strain H37Rv was observed, but was not pronounced, and the present findings are in keeping with the increasingly held view that H37Ra should be regarded as a low-virulence or attenuated strain rather than truly avirulent.