We tested the human papillomavirus 16 (HPV 16) early genes for their ability to immortalize or transform primary rat brain cells, using the expression plasmids that contain the neomycin-resistance-inducing unit (pSV2neo) and the transcriptional unit for the HPV 16 subgenomic DNA fragments controlled by the SV40 early promoter. After transfection, drug-resistant colonies were maintained by refeeding and replating for characterization. The E7 gene alone was found to be capable of immortalizing and morphologically transforming primary rat cells, and the transformed cells showed anchorage-independent growth. Although its activity was lower than that of the E7 gene, the E6 gene also immortalized primary rat cells. The immortalized or transformed cells contained HPV 16-specific DNA and mRNA.