The infectivity of Agrobacterium tumefaciens strain B6 irradiated with short-wavelength ultraviolet light was followed as a function of dose. Previously reported enhancements of B6 infectivity by ultraviolet irradiation, in samples inoculated after 1.75 hr of dark incubation at 27 C, or immediately following irradiation, were found to occur most frequently after losses in cell viability of 60% and of 90% or more, respectively. Changes in colony-forming ability and tumor-initiating ability with increasing dose showed no obvious correlation until the maximal infectivity promotion of samples inoculated immediately after irradiation was reached. Thereafter, both bacterial responses typically decreased in parallel. With low dose rates, infectivity promotions were obtained with less than 10% loss in cell viability. Data for tumor appearance and tumor growth resulting from inoculations with irradiated cultures showed no significant differences from controls, nor did the age of the bacterial culture or age of the host plant influence the response. The infectivity promotion appears to result from an increase in the proportion of viable cells that will subsequently initiate tumors. The characteristics of this ultraviolet infectivity promotion are shown to be most similar to those found in prophage and bacteriocin induction.