Mutational analysis of p21ras has shown that plasma membrane targeting requires the combination of a CAAX motif with a polybasic domain of six lysine residues or a nearby palmitoylation site. However, it is not known from these studies whether these signals alone target p21ras to the plasma membrane. We now show that these C-terminal sequences are sufficient to target a heterologous cytosolic protein to the plasma membrane. Interestingly, the key feature of the p21K-ras(B) polybasic domain appears to be a positive charge, since a polyarginine domain can function as a plasma membrane targeting motif in conjunction with the CAAX box and p21K-ras(B) with the polylysine domain replaced by arginines is biologically active. Since some ras-related proteins are modified by geranylgeranyl rather than farnesyl we have investigated whether modification of p21ras with geranylgeranyl affects its subcellular localization. Geranylgeranyl can substitute for farnesyl in combining with a polybasic domain to target p21K-ras(B) to the plasma membrane, but such geranylgeranylated proteins are more tightly bound to the membrane. This increased avidity of binding is presumably due to the extra length of the geranylgeranyl alkyl chain.