Abstract: Monoclonal antibodies reactive with oncogenic and activated ras p21 proteins containing aspartic acid or valine at position 13 and unreactive with normal ras p21 proteins containing glycine at position 13. The antibodies are secreted by hybridomas obtained by immunizing mice with synthetic peptides corresponding in amino acid sequence to positions 5-9 of normal ras p21 proteins, except having aspartic acid in place of glycine at position 13 and except for the addition of cysteine at the amino terminal end of the peptide. The antibodies and immunoreactive fragments are useful in the classification of malignant and premalignant lesions.