This part of ISO 5667 gives practical guidance on sampling, pre-treatment, performance and evaluation of environmental samples in the context of performing biological tests. Information is given on how to cope with the problems for biotesting arising from the sample and the suitability of the test design. It is intended to convey practical experience concerning precautions to be taken by describing methods successfully proven to solve or to circumvent some of the experimental problems of biotesting of waters. Reference has been made as far as possible to existing International Standards and guidelines. Information taken from published papers or oral communication is utilized as well. Primarily dealt with are substance-related problems concerning sampling and pre-treatment of environmental samples (e.g. waste water samples) for the performance of biotests. Special emphasis is laid on ecotoxicological testing with organisms (single-species biotests). Some features addressed in this general guidance apply as well to biotests using single-cell systems (in vitro biotest) and biodegradation studies as far as sampling and sample preparations are concerned. Testing of substances in the water solubility range is also addressed. This part of ISO 5667 is applicable to biological tests for determining the effect of environmental samples like treated communal and industrial waste water, fresh water, aqueous extracts (e.g. leachates, eluates), pore water of sediments and whole sediments. This part of ISO 5667 is also applicable to chemical substances. This part of ISO 5667 is not applicable to bacteriological examination of water. Appropriate methods are described in other International Standards.