In 239 apparently healthy subjects the 24-h urinary excretion of albumin transferrin, haptoglobin, IgM, IgG, IgA, immunoglobulin-free lambda and kappa light chains, lysozyme, and beta-2-microglobulin was studied by means of an automated immunoprecipitin reaction. The 24-h excretion of the proteins showed a very uneven distribution. Albumin was excreted in the largest quantities, 1.6-34.2 mg/24 h (0.95 range), and beta-2-microglobulin in the smallest quantities, 0-0.14 mg/24 h (0.95 range). Seven of 10 proteins were excreted in significantly lower quantities in children than in adults.