The alternatively spliced from mRNA of platelet glycoprotein IIb (GPIIb) with a deletion of exon 28 (GPIIb-28) has been isolated from the HEL cell cDNA library. The defective expression on the surface of DNA cotransfected COS-1 cells with GPIIb-28 and GPIIIa cDNAs was described in an earlier report. We studied siblings with Glanzmann's thrombasthenia who expressed only the GPIIb-28 mRNA in their platelets. Flow cytometry showed that the patients' platelets failed to bind GPIIb/IIIa complex-specific and GPIIb-specific monoclonal antibody. Western blotting showed that the patients' platelets had defective GPIIb and have trace amounts of GPIIIa. Sequence analysis was performed after polymerase chain amplification of the patients' GPIIb and GPIIIa mRNAs. The patients' GPIIb cDNA had a deletion of the exon 28 nucleotides. The polymerase chain reaction (PCR) from exon 27 to 29 showed that the GPIIb-28 mRNA was 3% +/- 1.6% of the normally, spliced form in control platelets, and 61% in the megakaryoblastic cell line UT-7. The patients' platelets showed only the GPIIb-28. Family study and quantitative PCR studies showed that these patients were compound heterozygotes of two GPIIb gene defects. The father's allele is described in this report and involves skipping exon 28 secondary to a base substitution at codon Gln948, CAG-->TAG. The mother's allele appears to involve decreasing GPIIb mRNA levels in platelets. Our results indicate that the GPIIb-28 is not expressed on platelet membranes as a stable GPIIb/IIIa heterodimer or left as a monomer in platelets. Our studies confirm the previous data observed in COS-1 cells expressing recombinant GPIIb-28.