Mixed species feeding assays were undertaken with pollution sensitive (Gammarus pulex) and tolerant (Asellus aquaticus) macro-invertebrates during August 2003 and April 2004. The purpose of this study was to establish if a test animals' response is comparable during in-situ and ex-situ toxicity tests. Seven test sites were established along an undisclosed stream, which received leachate discharge from an unlined, disused UK landfill site. Sampling points A–B were upstream of the contamination, C was adjacent to the influx and D–G were downstream of the leachate discharge (at 100 m intervals). During the in-situ and ex-situ tests, 2-week-old male laboratory bred A. aquaticus and G. pulex were used as test animals. The animals were transplanted to the seven sampling points for the duration of the in-situ tests, whilst water samples from each site were returned to the laboratory for ex-situ testing. The results show that the animals' mortality and feeding rates followed similar trends during the in-situ and ex-situ tests, however, the animals' response was amplified during the in-situ tests. It was also observed that the effects were greater in April, compared to August that may be attributed to a higher frequency of rainfall during spring, which could have flushed a greater proportion of the contaminant load from the waste mass and as a consequence, higher levels of pollution may have leached into the stream from the landfill site. The study, therefore, concludes that in-situ toxicity tests are a more precise monitoring technique, in comparison to ex-situ assays.