The involvement of toxic oxygen intermediates in the bacteriostatic effect of milk was determined by producing bacterial growth curves using turbidimetry in the presence and absence of oxygen radical-scavenging substances. Using whey as substrate, catalase, haemoglobin combined with ascorbic acid and xanthine oxidase inhibitors all provided protection against oxygen toxicity for a strain of Staphylococcus aureus and of Streptococcus agalactiae. Superoxide dismutase and mannitol were less effective. This was evident in whey alone and in the presence of oxygen radicals produced exogenously by the t-butylhydroperoxide, H2O2 and xanthine/xanthine oxidase systems.