The isolation and expression of a full-length cDNA clone encoding the B cell-specific glycoprotein CD19 is reported. The sequence of the cDNA predicts a glycosylated integral membrane protein with a precursor molecular weight of 51.8 x 10(3) and an extracellular domain organized into three contiguous Ig-like sub-domains. The cytoplasmic domain bears significant relatedness to two proteins encoded by the Epstein-Barr virus and the int-1 oncogene. CD19 transcripts are restricted to members of the B cell lineage, being most abundant in pre-B cell lines and least abundant in plasmacytomas.