There has been considerable speculation regarding possible imteractlons within biological systems between synthetic environmental pollutants and infectious disease agents (1-11). This type of interaction has been studied in our laboratory using the mallard, Anas platyrhynchos, as a test species (5,6). Since adult ducks are refractory to the overt manifestations of duck virus hepatitis (DVH) (12) and because duck hepatitis virus (DHV) has a predilection for the liver, the organ responsible for the metabolism of many chemicals, (13) an experiment was undertaken to determine if infection with DHV had any effect on the toxicity to adult mallards of p,p'-DDT or technical grade dieldrin.