Human 90K, also known as Mac-2 binding protein (Mac-2 BP), is a secreted glycoprotein which is widely expressed, binds to the human macrophage-associated lectin Mac-2, and may have a role in host defence. We have measured the concentrations of 90K in human breast milk from eight healthy mothers delivering mature healthy infants after full-term pregnancy using a specific immunoenzymatic assay. Maximal 90K concentrations were observed on days 2-3 post-partum, and ranged from 13.4 to 79.2 microg/ml. The concentrations of 90K in the milk had no correlation with those in the maternal blood.