Human body fluids such as mixed saliva, erythrocyte, plasma and mature breast milk were analysed for selenium (Se) and Se-dependent glutathione peroxidase (GSH-Px), which is the only active form of Se known in man. Selenium-dependent GSH-Px activity was detected for the first time in human mixed saliva. Body fluid GSH-Px, Se and protein contents expressed in terms of volume increased in the order, saliva less than milk less than plasma less than erythrocyte. However, the sequence of increase for GSH-Px (U/mg protein) and GSH-Px-bound Se (%) was plasma less than milk less than erythrocyte less than saliva, and that for Se (ng/mg protein) was erythrocyte less than saliva less than plasma less than milk. Significant positive correlations were found between GSH-Px and Se contents and between protein and Se contents expressed per volume for human saliva, erythrocyte and the whole human fluids investigated. Positive correlations between erythrocyte and plasma Se (ng/mg protein) and between plasma and saliva GSH-Px-bound Se (%) were also significant.