Two different mapping approaches were used to determine the human chromosomal location of the gene for protein S. A human protein S cDNA was used as a hybridization probe to analyze a panel of somatic cell hybrids containing different human chromosomes. Cosegregation of protein S-specific DNA restriction fragments with human chromosome 3 was observed. Three cell hybrids containing only a portion of chromosome 3 were analyzed in order to further localize protein S. Based on the somatic cell hybrid analysis, protein S is assigned to a region of chromosome 3 that contains a small part of the long arm and short arm of the chromosome including the centromere (3p21----3q21). In situ hybridization of the protein S cDNA probe to human metaphase chromosomes permitted a precise localization of protein S to the region of chromosome 3 immediately surrounding the centromere (3p11.1----3q11.2). Protein S is the first protein involved in blood coagulation that has been mapped to human chromosome 3.