The concentration of urogastrone (URO) and lactose was measured in breast secretions from four women. URO levels were high in precolostrum and fell markedly around the time of parturition to relatively constant low levels in mature milk. When milk samples were fractionated on a Biogel P10 column, the major peak of immunoreactivity occurred at the point at which pure URO eluted.There was good correspondence between the fall in URO levels at birth and the increase in lactose levels, suggesting that the decrease in URO was a dilution effect as the osmotic influence of lactose increased.