This chapter summarizes the understanding of the mechanisms of homogeneous enzyme-immunoassay, particularly with regard to the control of the activity of enzyme conjugates of haptens and antigens. The sensitivity of the assay is determined by four factors: the detectability of enzyme activity, the fractional change in activity upon binding of antibody to the enzyme-antigen conjugate, the binding constant for the antibody-antigen interaction, and the susceptibility of the assay to interferants in the sample such as endogenous enzyme, cross-reacting antigens, or enzyme inhibitors. Steric exclusion of substrate binding by antibody molecules directed towards surface antigens on the enzyme provides the simplest means of antibody mediation of enzyme activity. The chapter summarizes the enzymes that are capable of eliciting antibodies which are classified into four distinguishable groups according to the effect exerted upon enzymatic activity; namely, inhibitory antibodies, activating antibodies, protecting antibodies, and benign.