The Y13-238 monoclonal antibody reacts with human, mouse, and rat v-H-ras (within amino acids 120-138) and does not react with v-K-ras. v-H-ras binds GTP/GDP and has intrinsic GTPase activity. Ras proteins alternate between an inactive form bound to GDP and an active form bound to GTP, activated by a guanine nucleotide-exchange factor (GEF) and inactivated by a GTPase-activating protein (GAP). Under normal conditions, Ras family members influence cell growth and differentiation events. Mutations in the Ras family of proto-oncogenes are very common, being found in 20% to 30% of all human tumors. Ras point mutations are the single most common abnormality of human proto-oncogenes. The Y13-238 antibody does not neutralize ras GTPase binding and hydrolysis in vivo or in vitro.