C3 consumption and site formation was studied with specific antisera against three antigenic determinants of the C3 globulin molecule, namely, [β1C] or B, the β1A and the α2D determinant. Interaction between C3 in serum and optimal specific precipitates results in the rapid loss of the [β1C] or B determinant and concomitant appearance of C3i and free α2D in the fluid phase, the latter determinant being nearly unexposed in intact C3 globulin. Experimental data further suggest that the B determinant of the intact C3 molecule is lost by destruction or by intramolecular rearrangement in the C3 molecule after interaction with cell bound C. This is borne out by the following data. (a) Immune aggregates with bound complement are incapable of inducing anti-[β1C] antibodies in rabbits; (b) sensitized Lea-positive red cells with bound complement agglutinate with monospecific anti-α2D and anti-β1A, but fail to agglutinate with anti-[β1C]; (c) EAC1̄, cells prepared with limited C3 fail to agglutinate with anti-[β1C] but agglutinate strongly with anti-β1A and anti-α2D.