CD33 monoclonal antibodies recognize a 67-kD glycoprotein of unknown function that is expressed by early myeloid progenitors and their leukemic counterparts. We report here the cloning of the murine homolog of the human CD33 antigen. Two cDNA clones, differing by an 83- nucleotide insertion in the cytoplasmic region, were isolated. The insertion generated a shift in the reading frame within the cytoplasmic tail, resulting in two mouse CD33 isoforms, m33-A and m33-B, with distinct cytoplasmic domains and with predicted protein core molecular weights of 37 kD and 45 kD, respectively. The cDNAs and deduced amino acid sequences show extensive similarity with the human CD33 sequence with the highest homology occurring in the first and second lg-like domains (61% amino acid identity). The most significant divergence between the human and murine proteins occurs in their cytoplasmic portions. The murine CD33 mRNAs were detected in bone marrow, spleen, thymus, brain, liver, the multipotential progenitor cell line, A4, the myelomonocytic cell line, WEHI3B, the myeloid cell line, M1, and the macrophage cell line, P388, by Northern blot analysis. The expression pattern of the murine CD33 homolog suggests that the function of CD33 antigen in hematopoiesis may be conserved between humans and mice.