In situ heterotrophic uptake of mixed(14)C-amino acids and direct viable cell (DVC) count of Chesapeake Bay water samples were not significantly affected by the insecticide Kepone at concentrations [Symbol: see text]0.01 mg/1. Maximum inhibition of heterotrophic uptake,ca. 85-90%, and DVC count, 45-97%, was evident at concentrations of Kepone exceeding 0.2 mg/1. A specific activity index (Metabolic Activity/DVC or Kepone-resistant DVC), heterotrophic uptake, and DVC count were found to be statistically correlated (a=0.05) to one another, but negatively correlated with concentration of Kepone. The direct viable cell count proved to be a rapid, simple method for estimating the effect of Kepone on in situ estuarine microbial activity.