The first part of this project, dealing with in vitro studies, was published in 1985 (Ashby et al., 1985) and was summarized in Environmental Health Criteria 47 (WHO, 1985). The second part, which is the subject of this report, was published in 1988 (Ashby et al., 1988a).The need for inter-laboratory collaborative studies on an international scale arose from the necessity t investigate the value of short-term tests for detecting mutagenic and carcinogenic chemicals.