Mutations in ras oncogenes and expression of their encoded p21 protein products are believed to play an important role in carcinogenesis in humans. Detection of mutant p21 proteins in serum may be a useful molecular epidemiologic biomarker with which to study this process, and workers with heavy exposure to vinyl chloride (VC) represent a model population for such study. We studied the occurrence of a specific ras mutation (Asp 13 c-Ki-ras) by oligonucleotide hybridization and the expression of the corresponding p21 protein in tumor tissue and serum by immunohistochemistry and immunoblotting with monoclonal antibodies in five individuals with heavy exposure to VC and resultant angiosarcomas of the liver (ASL). Four of five (80 percent) of the cases of ASL were found to contain the mutation and to express the corresponding mutant protein in their tumor tissue and serum. Serum expression of the mutant protein also was examined in nine VC-exposed workers with liver angiomas and 45 VC-exposed workers with no evidence of liver neoplasia; eight of nine (89 percent) of the former and 22 of 45 (49 percent) of the latter were also positive for the mutant p21 in their serum. However, serum immunoblotting results for 28 age-gender-race matched, unexposed controls were all negative. Stratification by years of VC exposure showed a significant linear trend (P < 10(-5)) for the occurrence of the serum mutant p21 protein with increasing duration of exposure. These results suggest that detection of serum mutant p21 protein can be a valid surrogate for ras gene expression at the tissue level.(ABSTRACT TRUNCATED AT 250 WORDS)