The platelet membrane glycoproteins IIb (GpIIb) and GpIIIa form calcium-dependent heterodimers containing binding sites for fibrinogen, von Willebrand factor, and fibronectin. Although GpIIb and GpIIIa are distinct proteins, both GpIIb and GpIIIa are deficient in platelets from individuals with the recessive disorder Glanzmann's thrombasthenia. To gain a better understanding of the genetic basis for GpIIb and GpIIIa synthesis, we studied their synthesis by two human leukemia cell lines, HEL and K562. HEL cells contained complexes of GpIIb and GpIIIa, and K562 cells expressed GpIIIa, but not GpIIb, when stimulated with phorbol-12-myristate-13-acetate (PMA). RNA from HEL cells directed the in vitro synthesis of a 110,000-Mr precursor for GpIIb and a 92,000-Mr precursor for GpIIIa, which indicates that the synthesis of GpIIb and GpIIIa by HEL cells is directed by separate mRNAs. In contrast, RNA from PMA-stimulated K562 cells only directed the synthesis of a 92,000-Mr precursor for GpIIIa. The dissociation of GpIIb and GpIIIa synthesis in K562 cells suggests that GpIIb and GpIIIa may be the products of separate genes.