Studies were initiated on oncogene activation in two bivalve species with high frequencies of histologically identifiable gonadal neoplasms. Pathological assessments identified epizootic seminomas and dysgerminomas in softshell clams (Mya arenaria) from three Maine estuarine sites contaminated by herbicides and in hardshell clams (Mercenaria) from the Indian River in Florida, an area of potential citrus agrochemical exposure. NIH3T3 transfection assays were used to examine DNA isolated from these molluscan tumors for the presence of activated oncogenes. DNAs isolated from advanced tumors in both species were able to transform NIH3T3 cells in a standard focus assay. These same cells were also able to form colonies in low concentrations of serum and induce tumors in athymic mice. Cells expanded from isolated foci demonstrated anchorage-independent growth in soft agar. The results of these studies indicate that DNA from the clam tumors is able to transform mouse fibroblasts, which suggests that a transforming gene is present in these tumor cells. Studies are under way to identify the gene(s) detected by these assays.