Mutation and inactivation of H. influenzae have been measured following irradiation at various near-UV wavelengths. Inactivation takes place most readily at 334 nm (but is unaffected by absence of excision or postreplication repair), and decreases markedly at longer wavelengths. No induced mutations to resistance to novobiocin or streptomycin or to ability to utilize protoporphyrin instead of hemin were detected at any of the wavelengths used. There were also no detectable induced mutations in an excision-defective strain after 334-nm irradiation. These results are in contrast to the in vitro mutation of purified transforming DNA we previously observed.