From a cDNA library in lambda gt11 derived from poly(A)+ mRNA of human ovarian granulosa cells, a cDNA clone lambda HG12.1, containing an EcoRI insert of 470 bp, was identified. After subcloning of the insert into pUC18, the clone pHG12.1 was obtained and sequenced. The 5'-region of the insert of pHG12.1 was extended by the polymerase chain reaction (PCR) with cloned total cDNA. Assembly of the PCR fragment with the insert of pHG12.1 yielded clone pHG12. From the first open reading frame of pHG12 the amino acid sequence for a polypeptide of 25 amino acid residues (designated HG12) was derived, which was identical in 22 residues with yeast ribosomal protein YL41. It is therefore assumed that HG12 is the first mammalian homolog of yeast ribosomal protein YL41. Transcription of DNA fragments containing the coding region of pHG12 cloned into BluescriptM13, followed by cell-free translation, yielded a polypeptide with an apparent mol.wt. of 14.5 kDa, much larger than the theoretical mol.wt. (3454 Da). The discrepancy between theoretical and apparent mol.wt. was also observed for yeast ribosomal protein YL41. Southern analysis revealed that HG12 is not specified by a single copy gene. Homology for HG12 specific sequences is observed for bovine, porcine and rat species.