Enhanced yields of UV-induced back mutants to prototrophy are observed when irradiated cells of the Salmonella typhimurium frameshift strain LT2 hisC3076 (R46) are plated on defined medium containing broth (2.5%, v/v) rather than a trace (0.2 micrograms/ml) of the required nutrient (histidine). This broth effect is not abolished, and is in fact augmented, in an excision-deficient derivative of hisC3076 (R46) carrying the uvr-302 mutation. Since similar broth effects on UV-induced base-pair substitution mutagenesis have usually been attributed to inhibition of mutation frequency decline (MFD), and since MFD is in turn thought to reflect the activity of an intact excision-repair system, we sought to determine whether or not UV-induced pre-mutational lesions leading to the production of frameshifts are susceptible to MFD. Results with the double auxotrophic strain LT2 hisC3076 leuA150 (pKM101) showed that in a population of cells actually undergoing MFD (as judged by a rapid loss of UV-induced reversions of the base-pair substitution marker leuA150), no concomitant loss of UV-induced reversions of the frameshift hisC3076 marker could be detected.