Chimerism in humans is a rare phenomenon often initially identified in the resolution of an ABO blood type discrepancy. We report a dispermic chimera who presented with mixed field in his B antigen typing that might have been mistaken for the B3 subtype. The propositus is a healthy Korean male blood donor. Neither his clinical history nor initial molecular investigation of his ABO gene explained his mixed field agglutination with murine anti-B. Chimerism was suspected, and 9 short tandem repeat (STR) loci were analyzed on DNA extracted from blood, buccal swabs, and hair from this donor and on DNA isolated from peripheral blood lymphocytes from his parents. The propositus' red blood cells demonstrated mixed field agglutination with anti-B. Exon 6 and 7 and flanking intronic regions of his ABO gene were sequenced and revealed an O01/O02 genotype. B allele haplotype-specific PCR, along with exon 6 and 7 cloning and sequencing demonstrated a third ABO allele, B101. Four STR loci demonstrated a pattern consistent with a double paternal chromosome contribution in the propositus, thus confirming chimerism. His karyotype revealed a mosaic pattern: 32/50 metaphases were 46,XY and 18/50 metaphases demonstrated 47,XYY.