The lactoperoxidase (LPO) activity in guinea-pig milk and saliva has been investigated in sows suckling normal young, and young orally infected with Escherichia coli. There was a 5-fold increase in activity in milk during the 3--4 weeks of lactation; infection of the young did not alter this. There was no comparable increase in lactoperoxidase activity of saliva during this same period, either in the infected or non-infected group. The antibacterial activity of milk from sows suckling normal young increased with the lactoperoxidase, and this bactericidal activity could be reversed by LPO inhibitors such as penicillamine and cysteine but not by addition of sufficient iron to saturate the lactoferrin. In milk from sows suckling infected young, bacteriostatic activity occurring in samples from about 14 days after infection needed iron or both iron and penicillamine (or cysteine) for reversal, indicating that both the antibody-lactoferrin system and the LPO system may be involved in the infected state.