Bacterial counts on air samples and colony counts on settling plates revealed markedly fewer viable bacteria in a vertical laminar-air-flow operating room than in control operating rooms. In a fifty-four-month study, the postoperative infection rate was 0.79 per cent after 3,407 operations in the vertical laminar-air-flow operating room, and 1.04 per cent after 8,253 operations in the two control operating rooms. For the observed difference in infection rate to be statistically significant at the p ~ 0.05 level, there would have to be 9,917 operations using the laminar-air-flow system and 24,016 operations done in control rooms.