We have analysed the bacteriological quality of milk donated to the Oxford milk bank: we have studied the effects on bacteriology of simple additional antisepsis during the collection procedure and of Holder pasteurisation in a purpose built human milk pasteuriser. 12 pools of human drip breast milk (each comprising 75-120 24 hour milk samples) were studied: 6 pools were collected into vessels washed in detergent (Group I): in the other 6 a hypochlorite sterilising agent was used (Group II). Potentially pathogenic organisms were grown in untreated milk from both group I & II pools and included E.coli, S.aureus and Group B B-haemolytic streptococci. 7 species of non-pathogenic organisms were identified from both group I & II pools. Pasteurisation eliminated all potential pathogens from both groups, but did not reliably remove any of the non-pathogenic species. The collection of milk in hypochlorite sterilised vessels (Group II), resulted in a significantly lower bacterial count of both pathogenic and non-pathogenic species before pasteurisation; and significantly increased the chance of pasteurisation giving a sterile product. The potential effects of bacterial contamination on milk quality, and the production of toxic bacterial products in milk are discussed. We recommend post-pasteurisation monitoring for specified pathogens and immuno-diffusion testing for staphylococcal exterotoxin. We conclude that for pooled human milk, attention to the sterility of collecting vessels, together with precise Holder pasteurisation, results in a bacteriologically safe product which retains many of the protective properties of raw milk.