Infection of non-UV-irradiated cells of Salmonella typhimurium with UV-damaged P22 or KB1 phage induces rec A-dependent inhibition of cell division, cell mutagenesis and prophage induction but not inhibition of respiration. On the contrary, respiration and ATP concentration are increased after treatment with UV-damaged phage in both RecA+and RecA-strains, showing that this increase is not recA-dependent. Furthermore, infection with UV-damaged phage prevents both inhibition of respiration and decrease in ATP level in the UV-irradiated RecA+strain. This indirect induction of SOS functions is related to degradation of phage DNA as well as to the multiplicity of infection used, suggesting that DNA degradation may play an important role in the mechanism of expression of the SOS system. Our results give also support to the hypothesis that there exists a differentiation in the expression of the various SOS functions. © 1983 H. Veenman & Zonen B. V.