The expression of major histocompatibility complex class II or Ia antigens by neural cells has been investigated by indirect immunofluorescence on dissociated cultures from mouse cerebellum, cerebral cortex and dorsal root ganglia. Ia antigen expression was not detectable under standard culture conditions. However, treatment of mixed cultures from the cerebellum and of astrocyte cultures from the cerebral cortex with gamma-interferon preparations induced expression of Ia antigens on a fraction of the astrocytes. Under the same conditions, Ia+ cells could be observed in dorsal root ganglion cultures.