Cytosolic prenyl transferases from two human lymphoid tissue-derived cell lines, IM-9 and Molt-4 cells, are shown to isoprenylate recombinant p21H-ras. Isoprenylation was inhibited by an N-acetylated pentapeptide (N-Ac-Lys-Cys-Val-Leu-Ser), c, t-farnesyl diphosphate, c, t, t-geranylgeranyl diphosphate, t, t, t-geranylgeranyl diphosphate and a photolabile farnesyl diphosphate analogue. c, t-Farnesyl and t, t, t-geranylgeranyl monophosphates were also effective inhibitors of the Molt-4 enzyme but not the IM-9 enzyme. © 1991.