The biological activity of oxytetracycline hydrochloride (HCl) was shown to be very significantly reduced in the presence of fresh water sediment. The inhibition was a function of the interaction between the sediment and the agent and, over the range 2-8% w/v, its extent was proportional to the concentration of sediment. The biological activity of the agent was measured using a plate diffusion bioassay in which Yersinia ruckeri was used as the indicator organism and in tube-dilution MIC and MBC assays using Y. ruckeri and Staphylococcus aureus. The extent of the inhibition of the biological activity detected was dependent on both the bioassay method and on the indicator organism used. In the presence of 8% w/v sediment a maximum percentage biological activity of 15% was detected in plate diffusion bioassay experiments and a minimum of 0.3% was detected when the MBC values against S. aureus were examined, The mechanisms underlying the inhibition were not identified but evidence is presented that it was not mediated via the concentrations of Mg2+ or Ca2+ in the sediment. It is argued that the reduction of the biological activity of oxytetracycline HCl in the presence of river sediments presents major problems for the interpretation of data on the chemical concentration of this agent in such environments.