In 61 subjects with established or probable renal disease the concentrations of serum β2-microglobulin, as well as creatinine, were linearly related to inulin clearance when logarithmic scales were used. Comparatively poor linear regressions to inulin clearance were obtained for the serum levels of three other proteins, namely, the retinol-binding protein, lysozyme and α1-acid glycoprotein. Serum β2-microglobulin appeared to be more useful than serum creatinine for the detection of a slightly reduced glomerular filtration rate.