Molecular genetic analysis of 14 samples from unrelated individuals with the B(3) phenotype is reported here. Two different molecular changes in the blood group B gene were observed. One case was demonstrated to possess a 247G --> T mutation, which predicts an Asp83Tyr alteration. The B genes of the other 13 cases were shown to have a G --> A mutation at the +5 nucleotide of intron 3 (intervening sequence 3 [IVS3] + 5G --> A). Reverse transcription polymerase chain reaction analysis showed that the complete exon 1-exon 7 B transcript was absent, and transcripts that skipped exon 3 were instead present in the RNA sample from the B(3) individual with the IVS3 + 5G --> A mutation. The result shows that the IVS3 + 5G --> A mutation destroys the conserved sequence of the splice donor site and leads to the skipping of exon 3 during messenger RNA processing. The B(3) transcript without exon 3 predicts a B-transferase product that lacks 19 amino acids in the N-terminal segment.