AbstractRabbit polyclonal antibodies (pAb) were raised against a yeast ras-related protein YP20 and shown to be immunoreactive with human normal as well as altered Ha-ras and Ki-ias p21 gene products using immunoblotting and immunoprecipitation techniques. The p21 protein revealed by anti-YP20 antibodies comigrates with p21 protein detected by anti-p21 monoclonal antibody (Cetus Diagnostics). These pAbs were tested against a panel of human acetone-fixed tumor cell lines and malignant effusions and nonfixed fresh-frozen tissue sections obtained from cancer patients by the indirect immunofluorescence assay (IFA). Twelve of sixteen (75%) sarcoma and carcinoma cells lines and one fibroblast cell line were stained by the anti-YP20 pAb. The binding occurred most commonly in the cytoplasm. Six of eight fresh-frozen colon and breast cancer tissue sections were immunostained and normal sections from these organs or skin showed only low level of binding to thepAbs. Four of five malignant effusions were distinctively immunostained. These antibodies are suggested to serve as additional probes for assessing the expression of ras gene-related proteins in human malignancy.