The radiorespirometric technique, a modification of the heterotrophic activity assay, was evaluated as a monitor of toxic perturbation. The basis for the technique consists of trapping ¹⁴CO₂ evolved from the catabolism of a ¹⁴C-labeled substrate, and analyzing the resultant activity in a scintillation counter. An index of change in heterotrophic activity, the percent suppression, was calculated from the ratio of a toxified sample to a control. The effect of pentachlorophenol (PCP) on the heterotrophic activity of a laboratory-maintained aquatic culture was evaluated. The radiorespirometric technique detected changes in the heterotrophic activity for shorter exposure times and for PCP concentrations an order of magnitude lower than previously reported in the literature. Only 0.75 mg/L PCP caused approximately a 54 percent suppression of heterotrophic activity after a 30 min exposure. Radiolabeled glucose and glutamic acid were evaluated, and the radiorespirometric method was more sensitive at detecting changes in heterotrophic activity when the substrate used was glutamic acid. Whereas the error associated with the evolution and trapping of ¹⁴CO₂ apart from that introduced by microbial activity was only 13 to 20 percent, the variability induced by variations in the composition of the stock culture was quite high. The variability and lack of replicability of the heterotrophic activity experiments was the result of the heterogenous distribution of microorganisms and the alterations in the composition of the stock culture with time. The fit of the data to the first-order model of saturation kinetics was evaluated. The data derived during the study did not fit the first-order model probably because the added substrate concentrations were at trace levels. A protocol for the radiorespirometric technique is recommended. / M.S.