Peritoneal macrophages from mice infected with Bacille Calmette-Guérin (BCG) and from normal mice were examined for their effects in vitro on thymidine uptake by 10 murine lymphomas, a murine fibroblast line, and a guinea pig hepatoma. Only the murine fibroblast line showed growth inhibition in the presence of BCG macrophages. For the majority of tumors, normal macrophages were profoundly stimulatory to tumor cell DNA synthesis, while BCG macrophages were much less stimulatory, without being frankly inhibitory. The effect of 2-mercaptoethanol on tumor cell growth was also studied. All lymphomas stimulated to grow more rapidly in vitro by normal macrophages were stimulated to a similar degree by 2-mercaptoethanol.

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