The Rh-blood-group antigens (often described as Rhesus antigens) are associated with erythrocyte membrane proteins of approx. 30 kDa. We have determined the N-terminal 54 amino acid residues of the 30 kDa Rh D polypeptide (D30 polypeptide). We used primers based on these sequence data and the polymerase chain reaction (PCR) on human reticulocyte cDNA and genomic DNA to clone two types of PCR product of identical size. The two PCR products had related translated amino acid sequences between the 3' ends of the primers, one of which was identical with that found for the D30 polypeptide. We designate the two related mRNA species which gave rise to the PCR products as Rh30A and Rh30B, the latter corresponding to the D30 polypeptide. We have isolated cDNA clones for the Rh30A protein which encode a hydrophobic membrane protein of 417 amino acids. The Rh30A protein has the same N-terminal 41 amino acids as the D30 polypeptide, but beyond this point the sequence differs, but is clearly related. The Rh30A protein probably corresponds to the R6A32 polypeptide, another member of the Rh 30 kDa family of proteins, which may carry the C/c and/or E/e antigens. Hydropathy analysis suggests that the Rh30A protein has up to 12 transmembrane domains. Three of these domains are bordered by a novel cysteine-containing motif, which might signal substitutions at these cysteine residues. Information which supplements this paper (amino-acid-sequence-analysis histograms) is reported in Supplementary Publication SUP 50160 (4 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1990) 265, 5.