The environmental release of genetically altered microorganisms has prompted the investigation of their potential health effects by employing in vivo rodent models. Even though the direct health effects are addressed, the investigation primarily examines some potential indirect health effects. The model was used previously to examine several mutant Pseudomonas species isolated from a commerical product for polychlorinated biphenyl (PCB) biodegradation. Indirect effects examined include the colonization of the gastrointestinal tract, competition with the resident microbiota, and translocation of the dosed microorganisms to other organs . Ampicillin treatment of the mice enhanced the survival of the PCB degraders. Ampicillin treatment also had a significant effect on the microbiota in the intestine, by selecting primarily for the facultative species. The alteration of the lactose-fermenting enterobacteria, obligately anaerobic predominantly Gram-negative rods, and total aerobic and anaerobic populations were monitored in the presence and absence of the PCB-degrading pseudomonad. Depending on the dosed strain, a significant difference in the monitored populations as compared to control animals was observed. (Copyright (c) 1989 by the Society of Toxicology.)