The potential for bias, i.e., influences that cause results to deviate systematically from the truth is substantial both in toxicological research and in the performance of standardized toxicological testing. In this contribution, major potential sources of bias in toxicological research and testing are identified. Due to the lack of empirical studies of bias in toxicology, very little is known about its prevalence and impact. Areas to consider for such studies are pointed out, and it is suggested that such investigations should be given priority.