Influence of interferon gamma (IFN gamma) on astrocytes proliferating in response to unilateral injury of the cerebral hemisphere was investigated in 30-day-old rats. The brain injury was followed by an immediate injection of IFN gamma into the lesion cavity. On 1st or 2nd day following injury the animals were injected with [H-3]thymidine and killed 4 h after the injection. The proliferating astrocytes were visualised by combination of immunocytochemical staining for glial fibrillary acidic protein (GFAP) and autoradiography. Thereafter, numbers of GFAP-immunopositive and autoradiographically-labeled astrocytes located within the region of injury were counted. On the day 1 after injury no effect of lFN gamma administration was seen. However, on day 2 the average 43% reduction of the number of proliferating astrocytes was recorded. The reduction showed no dose-dependent changes. This in vivo evidence of IFN gamma-induced suppression of astrocyte proliferation was considered in relation to other determinants of astrocyte reactivity existing in the injured brain.