In the present study, we found that murine peritoneal macrophages elicited by BCG or Listeria monocytogenes release into the media an activity capable of stimulating the lung colonization as well as the expression of MHC class I antigens in B16 melanoma cells. A similar activity has previously been found in media conditioned by Corynebacterium parvum-elicited macrophages. Analysis by gel filtration chromatography of media conditioned by Corynebacterium parvum-, BCG- or Listeria monocytogenes-elicited macrophages revealed that the material responsible for the pro-clonogenic activity concentrated in chromatographic fractions corresponding to molecular weights (25 to 52 kDa) which are characteristic of certain cytokines. Thus, we challenged the various macrophage-conditioned media with polyclonal antibodies against IFNγ and TNFα, and found that the macrophage pro-clonogenic activity was completely abolished in the presence of anti-IFNγ antibodies, but only partially inhibited by anti-TNFα antibodies. This finding suggests a cooperative participation of the two cytokines to the pro-clonogenic activity of the media conditioned by Corynebacterium parvum-, BCG- or Listeria monocytogenes-elicited macrophages.