1. 123I-labelled β2-microglobulin (β2m) was injected into rats and protein or non protein-bound radioactivity was determined in plasma urine and several organs.

2. The observed kinetics differed from those expected according to the bicompartmental model for plasma protein turnover. The difference was attributed to the binding of a part of the tracer to a plasma component. Chromatography of plasma taken after injection of β2m showed an additional peak of radioactivity at the 55 000–80 000 daltons position.

3. In animals with ligated renal arteries, disappearance of the tracer from the plasma was markedly prolonged, and little non-protein-bound radioactivity was detectable in plasma, indicating that the kidney was the major site of catabolism of β2m.

4. Chromatography of plasma from control rats and rats with ligated renal arteries showed that the kidney was the major site of catabolism for free β2m only.

5. In normal rats, the urine was found to contain only non-protein-bound radioactivity.

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