Comparative methods were employed to investigate the relative biological activity of oxytetracycline in two media that have been used in studies of the frequency of oxytetracycline resistance in marine sediments associated with fish farms. Tryptone Soya Citrate Agar (TSCA) which has been used in Norwegian studies and 2216V agar which has been used in Irish studies were chosen for investigation. Using a nominal breakpoint concentration of 25 mu g ml(-1) oxytetracycline and identical incubation conditions higher total numbers and significantly lower frequencies of resistance were detected in ten samples of a non-fish farm marine sediment when 2216V media were used. Fifty seven of 60 colony forming units (95%), originally isolated on 2216V agar containing 25 mu g ml(-1) oxytetracycline were capable of forming colonies on TSCA, Of these, all could also form colonies on TSCA containing 25 mu g ml(-1) oxytetracycline. In contrast only 37 of 60 colonies originally isolated on TSCA containing 25 mu g ml(-1) oxytetracycline were capable of colony formation on 2216V agar and of these only 70% were capable of colony formation on this media containing 25 mu g ml(-1) oxytetracycline. The minimum inhibitory concentrations (MIC) of oxytetracycline against ten bacteria were established in Mueller Hinton Agar, TSCA and 2216V agar, MIC values were consistently higher when determined on both TSCA and 2216V agar but the extent of the increase showed significant strain to strain variation. These data indicate that it is not possible to arrive at a universally applicable value for the biological activity of oxytetracycline in these agar media.The significance of the inhibition of oxytetracycline activity in media that have been used to investigate the environmental impact of the use of this agent in marine fish farms is discussed.