We have isolated a transformation-suppressed cell line, R35, which is altered in the maturation of p21ras. It was obtained from the transformed cell line NIH3T3(rasEJ) as a revertant with flat morphology and anchorage dependent growth. Immunoprecipitation of steady state labeled ras proteins reveals that the revertant produces p21ras, and in addition, accumulates a protein that migrates as 23kD. Fusion of revertant cells with nontransformed rat2 cells yields cell hybrids which are retransformed and do not accumulate p23, suggesting that accumulation of p23 and suppression of transformation may be related. The accumulated p23 appears to be a precursor to p21 because (1) pulse labeling and cell fractionation of revertant cells shows that label chases from cytoplasmic p23 to membrane-associated p21, although at a slower rate, and (2) the p23 of revertant cells is indistinguishable from the p23 precursor of p21 in parent cells by partial V8 protease digestion and two dimensional electrophoresis. R35 cells are slower than their transformed parent in palmitylation and polyisoprenylation, but not in methylation of ras proteins. These results suggest that the transformation-suppressed cell line R35 is defective in the rate at which it posttranslationally processes ras proteins, possibly because it is slower in polyisoprenylation of the cytoplasmic p23 precursor protein.