The use of recombinant retroviruses expressing activated oncogenes to immortalize/transform fresh hematopoietic cells has been investigated. It was established that: a) fresh murine bone marrow (BM) cells can be immortalized in vitro in the absence of exogenous growth factors: b) immortalization of BM cells requires the collaboration of two oncogenes (v-myc and v-raf) each of which is ineffective by itself; c) under these conditions only macrophages are immortalized; and d) the immortalized macrophage cell lines express the functional activities of normal peritoneal macrophages. Further analysis of the mechanisms by which transforming oncogenes affect the cellular response to environmental stimuli was conducted on NIH 3T3 fibroblasts transformed by a single oncogene. We found that H-ras can inhibit the induction of c-fos mRNA and protein in fibroblasts stimulated by phorbol esters. However, only H-ras and not K- or N-ras, dbl or trk oncogenes affected the responsiveness to phorbol esters of NIH 3T3. These results provided the first evidence of the existence of a biological difference among ras oncogenes and indicated that at least some of the effects of H-ras may be due to reduced c-fos expression. The molecular events occurring during the activation of macrophages to a tumoricidal stage have been examined. We identified: a) stimulus-specific responses elicited by various activating agents which are evident during the early phases of macrophage activation, including augmentation of c-fos expression and of S- adenosylhomocysteine content; and b) common biochemical alterations expressed late during activation, namely a decrease in ribosomal RNA (rRNA) maturation and a consequent accumulation of rRNA precursors. We found that such rRNA precursors can activate double-stranded RNA-dependent enzymes via stable double-stranded secondary structures. We speculate that the interaction between these enzymes and the rRNA precursors plays a role in the activation and/or expression of tumoricidal activity by macrophages.