The observation that reduction of the inter-chain disulfides in rabbit antibody destroys its ability to interact with complement was confirmed and shown to be true also of human meyloma IgG1 subclass proteins. In the latter case a C1-binding assay was used. Further studies indicated that it was the interheavy chain disulfides which were essential for complement-binding activity: Non-covalently reassembled IgG (LHHL) was devoid of C1-fixing activity whereas molecules formed from covalently linked heavy chain dimers, and reduced and alkylated light chains (ie., LH-HL) were as active as the parent intact IgG. Fc fragments from IgG1 bound C1 and this activity was insensitive to the presence or absence of intact interchain disulfides. These bonds therefore are neither directly involved in C1 binding nor essential for the integrity of the binding site. We have also shown that although IgG4 does not bind C1, Fc fragments derived from this subclass fix C1 with an affinity comparable to that of the corresponding fragment from IgG1. These data suggest that quaternary interaction with other regions of the molecule (ie., Fab) may modulate the activity of the C1-binding site.