The bactericidal properties of human cell-free lymph have been investigated. Gram-negative and gram-positive bacteria are susceptible to the killing action of lymph fluid. The colony-forming ability of Escherichia coli and Bacillus subtilis was reduced by 3 logs after 20 min of incubation with 10% (vol/vol) lymph. Salmonella and Listeria were less susceptible to killing. The presence of multiple killing factors in lymph showing different specificities for different bacterial cells was demonstrated.