Two oligonucleotide primers were used for selective enzymatic amplification of a DNA segment encoding a major portion of the first constant region domain (CH1) of the human IgG1 heavy chain. The selective amplification was confirmed by use of subclass-specific oligonucleotide probes. Two 15-mer oligonucleotides, hybridizing with the alleles for the allotypes G1m(f) and (z), respectively, could then be used for determination at the genomic level of these two truly allelic allotypes. Serum and DNA samples from 12 individuals, one of them with a considerable amount of anti-Gm(f) antibodies, were used for allotype assignment by classical serological methods and by the new method operating at the genomic level. The resulting classifications agreed completely, demonstrating the reliability of the new method.