The study of tumors in fish has been hampered by the lack of suitable bioindicators especially because the ultimate diagnosis is not achieved until the fish are dead or killed. The present study was undertaken to develop simple, reliable and minimally invasive procedures to detect neoplasms in lake whitefish (Coregonus clupeaformis) and investigate their applicability as fish tumor biomarkers. Ethoxyresorufin-o-deethylase (EROD) assays have been widely used as an indicator of the activity of cytochrome P450-1A1, an isozyme located in a variety of tissues and in measurable amounts in the liver of many vertebrates. A fluorometric protocol using very small sized samples such as those from biopsies was used to measure EROD activity. Although no direct correlation could be made between EROD activity and liver tumor occurrence, EROD activity was significantly higher in livers with high melanomacrophage aggregates (P<0.001). Fibronectin (Fn) is a plasma and cellular protein that can be conveniently measured from biological fluids and has been used as tumor biomonitor in humans. Changes in Fn within serum and tissue samples of whitefish were monitored. Mean Fn levels in serum samples (n=65) was 2.03% of total serum proteins. Among the serologically evaluated fish, three had hepatic neoplasia as diagnosed by histological means. Fn levels in two of these were reduced at 1.89 and 1.22% of total serum proteins, however, no statistical correlations could be made with such small sample size, and further analysis is in progress.