Horseradish peroxidase is often used as an antibody-coupled enzyme and several procedures have been developed to obtain IgG-peroxidase conjugates. The most widely used are coupling with periodate or glutaraldehyde. To compare the efficiency of these methods, the authors conducted periodate coupling or glutaraldehyde coupling in one or two steps, using the same batches of peroxidase, C-reactive protein (CRP) and anti-CRP monoclonal antibodies to develop a specially sensitive Elisa for CRP. Comparison of immunoenzymatic activities showed that periodate-mediated conjugation was much more efficient, because the activity of the coupling products was about 100 times greater than that of the products obtained after one or two-step conjugation with glutaraldehyde. The lower coupling efficiency observed with glutaraldehyde was not due to inactivation of the coupling agent or to a possible decrease in the affinity of the conjugates for CRP due to the coupling procedure. The differences in efficiency can be ascribed to the fact that periodate induced more coupling sites than glutaraldehyde. Periodate is therefore a better coupling agent for preparing conjugates to be used in Elisa or related techniques, in which conjugate size does not hinder accessibility to the antigen.