IT has been shown that immunoglobulins of low molecular weight (micro-immunoglobulins) in normal human urine (compare ref. 1) are predominantly composed of material which closely resembles light polypeptide chains2. This is also true of the micro-immunoglobulins in “post-exercise” urine3 and in urine from patients with connective tissue disease4. Micro-immunoglobulins in normal human plasma5–7 include material resembling light chains2. Plasma and urine from patients with “heavy chain disease” contain large quantities of protein which is similar to the Fc fragment of IgG, a piece of heavy chains produced by papain digestion8. It has been reported that a small part of the micro-immunoglobulins in post-exercise urine3 and in urine from patients4 and normal individuals9 is related to the heavy chain and Fc fragment of IgG. The material related to Fc was not characterized in these reports. Recently, it was demonstrated that protein in normal urine includes material resembling the Fc′ fragment of IgG (ref. 10). Fc′ fragment released by digestion of IgG with papain or trypsin has a smaller molecular size than Fc and is antigenically deficient with respect to this fragment11.