The influence of gamma radiation, X-rays, UV-light at 254 nm and 365 nm, the latter combined with furocoumarin sensitizers has been studied on plaque forming ability, phage adsorption, DNA injection, and replication processes. UV-light (365 nm) plus furocoumarin treatment of phage particles gave rise to two types of DNA crosslinks. Type I crosslink corresponded to furocoumarin mediated covalent linkage between adjacent sites in opposite strands of the double helix. Crosslink type II (hairpin crosslink) required a highly condensed DNA and corresponded to the covalent linking of adjacent sites in double helical segments of a folded DNA molecule. The relationships of the type I crosslinks to inhibition of DNA replication and of the type II crosslinks to suppression of the DNA injection process are discussed. Pronounced deviations in phage inactivation have been obtained by X-ray radiation alone compared with UV-light (254 nm) pretreated and subsequently X-ray irradiated probes. The observed protective effect of the latter was described in terms of an inducible repair mechanism. The same protection has been observed by combination of gamma radiation with a sublethal UV-light (254 nm) dose.