1. A monoclonal antibody (subclass immunoglobulin G1) has been raised against human brain-type creatine kinase (CK-BB). This antibody did not cross-react with either muscle-type creatine kinase (CK-MM) or heart-type creatine kinase (CK-MB). 2. The binding constant measured with native antibody was 6 X 10(8) M-1. In the presence of 2mM-dithiothreitol this constant was some 40-50-fold greater. 3. Partial reduction and alkylation showed that the increased binding was due to a direct effect on the antibody and was associated with concomitant cleavage of the heavy-heavy interchain disulphide bonds. The binding constant measured with Fab' fragments produced from reduced and alkylated antibody was similar to that shown by the native, unreduced antibody. 4. The molecular weight of the complex found in the absence of mercaptans was consistent with one antibody and one CK-BB molecule, whereas the molecular weight estimated with reduced and alkylated antibody was consistent with a complex of two antibodies and two CK-BB molecules. 5. It is proposed that mercaptans increase the flexibility of the hinge region of the antibody molecule, allowing the formation of a higher-order complex with increased avidity for the CK-BB dimer.