Adult sheep were given into the rumen c. 10(8) cells each of donor strains of E. coli containing an R factor and prospective salmonella-recipient organisms and were maintained on a diet of lucerne chaff; the animals excreted the organisms, remained healthy, and no transfer of the R factor was detected. When the animals were starved for 48 h before inoculation, the ruminal environment was altered so that, on resumption of feeding, small numbers (c. 10(2)-10(4) cells--less than one cell per ml of rumen fluid) of the introduced organisms were able to multiply and reach sufficient numbers for the transfer of R factors to occur within the rumen. One animal, given 7-8 X 10(3) cells of recipient S. lomita after starvation for 48 h, became a carrier of this organism. A second animal, given 4-4 X 10(2) cells of S. typhimurium after starvation for 48 h, developed acute, fatal salmonellosis 5 days later; at the time of death, large numbers of salmonella organisms (c. 10(9) cells per g) were present in the faeces; these included many (c. 10(6) cells per g) that had received the R factor by transfer in vivo. These results indicate that short periods of starvation may enhance the transfer of R factors and possibly other plasmids between suitable micro-organisms in vivo, and may increase the susceptibility of animals to pathogenic micro-organisms.