The subunit complex C1r2C1s2 of the first component of complement was investigated by small-angle neutron scattering in both the activated and unactivated forms. From these experiments, a molecular weight of 390,000 for C1r2C1s2 was found. The matchpoint was determined to be 43% 2H2O. Both results are consistent with composition data. The partial specific volume is 0.751 ml/mg. The radius of gyration at infinite contrast was found to be 17 nm for C1r2C1s2 and 1.1 nm for the cross section. Models for C1r2C1s2 were computed by the method of hard spheres, in which C1r2C1s2 was represented by spheres 0.87 nm diameter arranged in a straight rod of length 59 nm and a circular cross section of 3.2 nm. This rod can be bent at one or two places by up to 60 degrees without significant effect on the calculated radii of gyration. The model is in agreement with published ultracentrifugation and electron microscopy data.