This paper reports the description of fragments of human C3, purified from aged sera. Sera were incubated at different temperatures (4 degrees, 20 degrees and 37 degrees C) for 8 days or more. At 4 degrees C, the major isolated fragment was C3b which showed a heterogeneity of structure. Under reducing conditions, it liberated a 40,000 dalton fragment which reacted against an anti-C3d and which we called "C3d-like". At 20 degrees C, C3b was partially destroyed to yield the C3c- and C3d-fragments. At 37 degrees C, C3d and C3c were obtained. C3c presented a structural heterogeneity. It was made either of two chains of 75,000 daltons or of three chains with 75,000, 38,000 and 31,000 dalton MW. The comparison of these fragments with those obtained by tryptic hydrolysis led us to propose a pathway to the degradation of C3 and to bring some informations about the structure of this molecule.