Cells from a dihydrofolate reductase-deficient Chinese hamster ovary cell line were hybridized to human fetal skin fibroblast cells. Nineteen dihydrofolate reductase-positive hybrid clones were isolated and characterized. Cytogenetic and biochemical analyses of these clones have shown that the human dihydrofolate reductase (DHFR) gene is located on chromosome 5. Three of these hybrid cell lines contained different terminal deletions of chromosome 5. An analysis of the breakpoints of these deletions has demonstrated that the DHFR gene resides in the q11----q22 region.