The position of genes coding for PGD, PPH1, UGPP, GuK1 PGM1, Pep-C, and FH on human chromosome 1 was investigated by analysis of karyotype and enzyme phenotypes in man-Chinese hamster somatic cell hybrids carrying aberrations involving chromosome 1. Suitable hybrid cell lines were obtained by X-irradiation of hybrid cells carrying an intact chromosome 1 and by fusion of human cells from a clonal population carrying a translocation involving chromosome 1 with Chinese hamster cells. The latter human cell population had been isolated following X-irradiation of primary Lesch-Nyhan fibroblasts. In addition, products of de novo chromosome breakage in the investigated hybrid lines were utilized. By integrating the results of these analyses with earlier findings in our laboratory, the following positions of genes are deduced: PGD and PPH1 in 1q36→1q34; PGM1 in 1p32; UGPP in 1q21→1q23; GuK1 in 1q31→1q42; Pep-C in lq42; and FH in 1qter