Kinetics of inactivation of transduction by phage P1bt which had been treated with ultraviolet light (UV) or nitrous acid (NA) was examined. With Escherichia coli B/r (radiation-resistant), low doses of UV increased transduction frequency, but the frequency was exponentially inactivated by higher doses. Little initial stimulus was observed in strain Bs−1 (radiation-sensitive). The final rate of decay was the same as in B/r. The initial stimulus of transduction in B/r was probably a consequence of increased recombination resulting from dark repair. It was estimated that another nucleotide within 1000 nucleotide pairs had to be damaged by UV to prevent a given nucleotide from successful transduction. The NA dose response was the same for the two strains. An initial stimulus of transduction was followed by exponential decline. The UV-repair enzymes missing in Bs−1 were not required for repair of NA-induced damage to transducing or lytic phage DNA. Low recovery of new mutations in the transductants showed that mutagen-induced damage to transducing DNA was excluded from recombinant chromosomes. The few recovered mutants may have resulted from “normal” error in recombination.