A method based on indirect conductimetry was developed for detecting the minimum effect concentration (MEC) of oxytetracycline and oxolinic acid on the metabolic activity of the microflora of freshwater sediments. For each agent, three repeat examinations of separate samples collected from the same river location generated identical MEC values (20 mg/kg for oxytetracycline and 0.63 mg/kg for oxolinic acid).Sediment microcosms were employed to determine the impact of these agents on the frequency of resistance in the culturable microflora. The minimum concentration capable of selecting for increased frequencies of resistance (MSC) was determined from resistance profiles constructed by measuring the percentage of resistant colony forming units over a range of selective concentrations. For each agent, the MSC values calculated in three independent analyses were identical to the previously determined MEC values. Comparisons of the resistance profiles generated from impacted and unimpacted microcosms allowed the establishment of empirically justifiable breakpoint concentrations for incorporation into Iso-Sensitest agar. These were 12.5 mg/l for oxytetracycline and 1.56 mg/l for oxolinic acid.Arguments are presented that the determination of MEC values for any sediment will allow the determination of no-effect concentrations that can be used in predictive environmental impact studies. (C) 2001 Elsevier Science B.V. All rights reserved.