In wounds on the back of guinea-pigs an inoculum of 6 x 10(5) of Escherichia coli and of Bacteroides fragilis produced a frank infection rate of 27 per cent. Addition of a catgut or a braided nylon tie statistically significantly increased the rate to 56 and 48 per cent respectively. Prolene and silk produced much smaller, non-significant rises. The rate with polyglycolic acid was 18 per cent which, though not statistically significant, suggests the possibility that this material produces conditions unfavourable to multiplication of bacteria that produce this type of infection.