We examined the induction of HPRT mutations in CHO cells exposed to low fluences of (238)Pu alpha particles from a specially constructed irradiator. The dose-response relationship was linear over the dose range of 5 cGy-1.2 Gy. However, unexpected sensitivity, leading to a significantly higher frequency of mutations than would be predicted by a back extrapolation from the data for higher doses, was observed in the dose range below 5 cGy, where the mean number of alpha-particle traversals per nucleus was significantly less than one (0.05-0.3). The frequency of mutations induced by a single alpha particle traversing the nucleus of a cell was increased nearly fivefold at the lowest fluence studied. The data are consistent with the conclusion that the enhanced efficiency of each nuclear traversal at low particle fluences is the result of mutations arising in nonirradiated, bystander cells.