1. A single, biologically active, highly purified preparation of the third component of complement (C3) has been used to study C3 metabolism in eleven normal subjects.
2. Half-life of the radioactive protein ranged from 64 to 81 h with fractional catabolic rates (FCR) of 1·36–1·95% of the plasma pool/h. Extravascular/intravascular pool ratios were 0·19–0·55.
3. Analysis of plasma and extravascular radioactivity curves suggested that the primary site of C3 metabolism was intravascular.