As a contribution to the second round of rodent carcinogenesis prediction organised by the U.S. National Institute of Environmental Health Sciences (NIEHS), speculative predictions for 30 chemicals currently under evaluation in U.S. National Toxicology Program (NTP) rodent bioassays are presented. Core chemical data received from NIEHS were supplemented by relevant information from commercially available scientific databases to provide input for reasoned assessment. For each chemical, carcinogenesis by a genotoxic or nongenotoxic mechanism or noncarcinogenesis is predicted; species-and target organ-specific predictions are also presented, together with arbitrary indices of confidence in each such prediction. In all or nearly all cases, an element of informed speculation was a necessary part of the prediction process, but the rationale for each decision is briefly described. It is predicted that ten chemicals will prove noncarcinogenic, five will be carcinogenic to mice only, and 15 will induce tumours in both species.