BACKGROUND Povidone-iodine solutions are widely used and highly effective antiseptics. Although commonly used at full strength, this concentration appears to be toxic to the cells involved in wound healing. Few systematic studies of povidone-iodine toxicity have been reported. The effects of various dilutions of 10% povidone-iodine solution on the growth of human diploid fibroblasts were assessed using in vitro cell culture. OBJECTIVE The purpose of this study was to systematically evaluate the toxicity of povidone-iodine on living cells using an in vitro model. METHODS Adult skin fibroblasts and fetal lung fibroblasts were subcultivated at various seeding densities of 3000-10,000 cells/cm2 and grown in polystyrene tissue culture flasks under an atmosphere containing 5% O2, 5% CO2, and 90% N2. Cells were grown in a medium containing various concentrations of povidone-iodine (1%, 0.1%, 0.025%, 0.01%, and 0%). Cell attachment was reduced by 0.1% and 1% povidone-iodine in our initial studies; subsequent experiments were performed by changing the medium to contain the povidone-iodine 24 hours after seeding. Growth curves were performed by counting triplicate cultures every 48 hours for 250-300 hours. RESULTS Fibroblast growth was progressively retarded at 0.01% and 0.025%, and totally inhibited by 0.1% and 1% povidone-iodine solutions. Partial recovery of cell growth after limited exposure of cultures to dilute solutions of povidone-iodine was noted. CONCLUSIONS This study shows that even dilute solutions of povidone-iodine are toxic to human fibroblasts. The results indicate that caution should be used when povidone-iodine is placed on an open wound, and that prolonged contact with viable uncontaminated tissue should be avoided.