A screening procedure which permits identification of compounds based on their activities against specific biological targets directly in a living organism, Saccharomyces cerevisiae, has been established as part of our new drug discovery programme. Use of this assay has provided the first direct evidence that TOP1 and RAD52 proteins are involved in the mode of action of bisdioxopiperazine ICRF compounds, which thus express a mode of action quite distinctive from the other known TOP2 inhibitors evaluated. The functional assay is based on a comparison of pairs of yeast differing in their phenotypes by specific traits: the expression or lack of expression of ectopic human DNA topoisomerase I, with or without that of the RAD52 gene. Amongst a series of anticancer agents, inhibitors of topoisomerase I (camptothecin) were identified as such in yeast expressing human topoisomerase I, whilst the presence or absence of RAD52 protein permitted the discrimination of compounds generating double-stranded DNA breaks, either directly (bleomycin) or involving DNA adduct formation (cisplatin), or indirectly with DNA damage mediated via inhibition of the topoisomerase II enzyme (etoposide). Notably, however, both the RAD52 protein and the lack of TOP1 enzyme appeared implicated in the cytotoxic activities of the series of bisdioxopiperazine ICRF compounds tested. This functional assay in a living organism therefore appears to provide a valuable tool for probing distinctive and specific mode(s) of action of diverse anticancer agents.