FKBP-12 is the major T cell binding protein for the immunosuppressive drugs FK506 and rapamycin. It is a member of the immunophilin family of proteins which are believed to play a role in immunoregulation and basic cellular processes involving protein folding and trafficking. The chromosomal assignment of the human FKBP-12 gene was determined by using the polymerase chain reaction to amplify an intron-containing region of the gene in purified DNA isolated from 42 human-rodent somatic cell hybrids. The results of this analysis indicated that the FKBP-12 gene resides on human chromosome 20.