A family of new microporous composite membranes that do not require post-immobilization blocking or quenching steps has been developed for diagnostic immunoassay applications. These membranes, Autobloc membranes, are shown to have low nonspecific binding of proteins. Immobilization of antibody or antigen to the membrane can be achieved either by covalent binding or by adsorption. The correlation between the initial immobilization capacity for IgG and the effective immobilization capacity of microporous membranes is discussed. The performance of Autobloc membranes is demonstrated by dot-enzyme-linked immunoassays in the flow-through mode with several commonly used enzyme-substrate systems and compared to the performance of other membranes. The advantages of using Autobloc membranes are suggested.