The antimicrobial effect of 20 minutes exposure to 10% povidone-iodine solution and to 5% neomycin-erythromycin solution was evaluated in vitro in 6 suspensions of dog feces. Povidone-iodine eliminated aerobic growth (P less than 0.001) and reduced anaerobes 4.01 +/- 1.06 (P less than 0.02); C. perfringens was the only anaerobic organism grown. Forty unprepared dogs underwent resection of the sigmoid colon and primary anastomosis. Twenty received normal saline and 20 povidone-iodine injected intraluminally immediately before resection. The colon contents of povidone-iodine treated dogs grew only 0.07 +/- 0.07 aerobes and 3.74 +/- 0.49 anaerobes (all Clostridia) (log10/ml colon contents) (P less than 0.001). All povidone-iodine dogs survived 3 weeks with no anastomotic leaks; three controls died from anastomotic leak within the first week (P = 0.12). Reexploration of survivors revealed less perianastomotic reaction in the povidone-iodine group. Twenty minutes exposure to povidone-iodine produced a significant decrease in bacterial counts in vitro and in unprepared sigmoid colon. No adverse effects were demonstrated. Images Fig. 4.