The ras p21 oncogene product migrates as a heterogeneous series of polypeptides as resolved by both one- and two-dimensional polyacrylamide gel electrophoresis (PAGE). We have prepared polyclonal rat serum antibody to ras p21 and used this as well as monoclonal antibodies to immunoprecipitate forms of p21 synthesized in vivo in transformed NIH3T3 cells. Two-dimensional PAGE of p21 resolved two distinct groups of polypeptides, one acidic (pI 4.8-5.3) which we call the "A" forms, and one less acidic or more basic (pI 6.5-7.0) which we call the "B" forms. It is the membrane-localized, B forms of v-ras-Ha p21 that are predominantly phosphorylated in vivo.