Milk protein concentrations were determined in five samples of neonatal milk (witch's milk) by either double antibody radioimmunoassay (IgA) or by single radial immunodiffusion (IgG, lactoferrin, lysozyme and albumin). Neonatal milk IgA concentrations ranged from 0.43 to 118.2 mg/l; the corresponding mean neonatal serum IgA concentration was 2.6 +/- s.e. 1.45 mg/l (n = 48). Sucrose density gradient ultracentrifugation indicated that the IgA detected was of the 11S type. IgG, lactoferrin, lysozyme and albumin were detected in neonatal milk in similar concentrations to those found in maternal milk.