Murine fibroblasts respond to mitogens by sequential gene expression in which immediate-early, or primary response, gene transcription factors direct expression of secondary transcripts encoded by delayed-early response (DER) genes. DER gene products include growth progression factors, but the products of several novel cDNAs are unknown. Murine and human cDNAs derived from one novel DER gene (DER12) were characterized to identify its product and to probe its role in the growth response. Both sequences encoded a hydrophobic 36 kD protein (HNP36) that was related to the yeast protein, FUN26. Anti-murine HNP36 antibodies were prepared and shown to immunoprecipitate both mammalian in vitro translation products. Immunocytochemical staining indicated localization of HNP36 to the nucleolus where its concentration increased after mitogen stimulation. Although HNP36 protein function is unknown, its identification as a nucleolar gene transcriptionally activated by growth factors implicates it as participating in the proliferative response.