The immediate-early (IE) genes of murine cytomegalovirus (MCMV) are expressed in the absence of prior viral protein synthesis and regulate the transcription of MCMV early genes. The effect of MCMV IE genes on growth induction was studied. Different plasmids containing MCMV IE genes were microinjected into arrested NIH 3T3 mouse fibroblasts. Plasmids containing the ieI gene coding for the 89,000-Mr major IE protein pp89 were found to stimulate the expression of the c-fos protooncogene. Synthesis of pp89 and its transport to the nucleus appeared to be required for c-fos expression. DNA synthesis occurred in cells that were injected with MCMV IE genes and in neighboring cells that were not injected. The results suggest that the phosphoprotein pp89 stimulates cells to enter the cell cycle.