SummarySerumβ2microglobulin (β2m) levels were determined together with serum creatinine and inulin clearance in 500 patients with various degrees of renal insufficiency, excluding patients with connective tissue disorders or lymphoproliferative disorders. The logarithmic relationship observed between Sβ2m and inulin clearance, was best fitted to experimental data whenβ2m values below and above 245 mg/l were treated separately. The plasma turnover of125Iβ2m was studied in two healthy adults, two anuric patients (one of them without kidneys) and eight patients with various degrees of renal insufficiency. Results were analyzed according to a bicompartmental model. The fractional metabolic rate was positively correlated with inulin clearance and little (<3.5 %) extrarenal catabolism was demonstrated. The half-life ofβ2m in intravascular compartment varied from 107 min. in normals to more than 39 hrs in the anephric patient.