Mouse alpha-fetoprotein (AFP) was added for 24 h to primary cultures of mechanically dissociated mouse brain fetal hemisphere cells grown in a serum-free medium. The intracellular presence of AFP could be demonstrated by immunocytochemical methods mainly in the cytoplasm of numerous differentiating neurons. The staining occurred only after 6 days in culture. The whole network of neurites, straight processes and filaments also appeared positively labeled. No AFP was noticeable in control cultures grown in the absence of the protein.