Supplementary formula feeds inhibited the protective effect of expressed raw and pasteurised human milk in 226 high-risk neonates in a randomised controlled trial. The infection rate in the group given pasteurised human milk and formula (33%) was significantly higher than the rates in the groups given raw human milk (10.5%), pasteurised human milk (14.3%), and raw human milk and formula (16%). This accords with the impressions that some of the association of infection with artificial feeding is partly attributable to the lack of the protective effect of human milk. Heating expressed human milk to 62.5 degrees C for 30 min significantly reduces its protective effect.