Two alleles of IL-3 have been reported to GenBank (GenBank M14743, M20137). The sequence difference between these two alleles is at the first nucleotide of the 27th codon (the 131st nucleotide from the initiation site): thymine and cytosine, and leading the amino acid difference: proline and serine (Pro27Ser). The other allelism, thymine and cytosine, was also observed at position -16 of the IL-3 upstream promotor region (GenBank L10616, M60870). We clarified that these substitutions were frequent polymorphisms in the Japanese population by using the mismatch-PCR (polymerase chain reaction)/RFLP (restriction fragment length polymorphism) method.