Culturable counts of antibiotic resistant, genetically engineered Pseudomonas fluorescens were determined on antibiotic-containing plate count agar during starvation in water. The mean of the colony counts on plate count agar with or without tetracycline (4.9 times ten to the sixth power/ml) was significantly higher than mean colony counts on plate count agar containing either nalidixic acid or nalidixic acid plus tetracycline (2.5 times ten to the sixth power/ml). After 20 days of starvation the highest mean colony counts continued to be obtained on plate count agar (7.2 times ten to the sixth power/ml) with slightly, but significantly, lower counts obtained on plate count agar containing either nalidixic acid (5.6 times ten to the sixth power/ml) or tetracylcline (1.5 times ten to the sixth power/ml). A combination of nalidixic acid and tetracycline in plate count agar, however, dramatically reduced colony counts (8.3 x 100/ml) after the starvation period. The addition of catalase to plate count agar containing nalidixic acid and tetracycline negated the effect caused by the combination.