In a previous publication (Bobrow et al., J. Immunol. Methods (1989) 279-285), we described a novel signal amplification method, catalyzed reporter deposition (CARD), and its application to microplate immunoassays. The method utilizes the analyte-dependent reporter enzyme (ADRE) to catalyze the deposition of additional reporter onto the surface of a solid-phase immunoassay system. In this paper, we describe the utilization of CARD amplification for nonradiometric membrane assays where detection is facilitated by the formation of an insoluble chromogenic product. In the examples described, deposition of reporter is accomplished in two steps: (i) a horseradish peroxidase (HRP) ADRE catalyzes the deposition of either a biotin or fluorescein labeled phenol, and (ii) incubation with either enzyme labeled streptavidin or anti-fluorescein, respectively, results in the deposition of additional enzyme. Using this method, we have improved detection limits from 8- to greater than 200-fold depending on the amplification format and the chromogen used.