Methylmalonic acidemia is an often fatal inborn error of organic acid metabolism due to deficiency of methylmalonyl-CoA mutase. The cloning of genes encoding this enzyme and the advent of technologies for gene transfer have introduced the possibility of somatic gene therapy for this disorder. Gene therapy may require replacement of the defective enzyme in hepatocytes, which have a greater capacity for propionate metabolism than other somatic cells and represent the principle physiological site of propionate metabolism. We describe construction of an amphotropic retroviral vector containing the human methylmalonyl-CoA mutase cDNA. This vector is shown to transduce primary MCM-deficient fibroblasts and restore levels of [14C]propionate metabolism by cultures of nonselected cells to normal. This vector will transduce primary human hepatocytes and direct transcription of recombinant human MCM from the integrated provirus. This work demonstrates the feasibility of retroviral-mediated gene transfer of methylmalonyl-CoA mutase into primary human cells, including hepatocytes which represent a difficult, but potentially necessary, target for gene therapy of methylmalonic acidemia.