The present study was conducted to determine the influence of two organochlorines, i.e., polychlorinated biphenyl (Aroclor 1242) and dieldrin, or RNA synthesis in elicited peritoneal macrophages. Tritiated uridine ([3H]uridine) incorporation into RNA was used as an index of RNA synthesis. RNA synthesis was depressed in macrophages incubated in the presence of dieldrin in both a time- and dose-related manner. The dieldrin-induced inhibition of RNA synthesis was further exacerbated when the macrophages were actively phagocytosing latex particles. Aroclor 1242 similarly inhibited RNA synthesis in resting macrophages; however, inhibition was not seen until a concentration exceeding 7 ppm of Aroclor 1242 was used in the incubation medium. Dieldrin-induced inhibition of RNA synthesis was observed at levels of 1 ppm. The inhibition of RNA synthesis provides a corroborative index to support the hypothesis previously advanced that the observed in vivo organochlorine-induced immune suppression may be macrophage mediated. Since the observed in vivo immune suppression was manifested in the absence of any morphological changes in the target lymphoid-macrophage tissues, a biochemical lesion, perhaps at the level of RNA synthesis, may be a site for the organochlorine-induced immunosuppression.