Using fluorescein-labelled S. typhi coated with C3b (FBC) the presence of a receptor for C3b in normal human glomeruli has been confirmed. A quantitative system, counting the number of FBC bound per unit area of glomerulus, has been developed. Experimental variables have been studied to determine optimal conditions for FBC binding. Glomerular FBC binding has been shown to be dependent on FBC concentration, temperature and time of tissue incubation. A standardized procedure has been adopted. Using this technique we have examined a number of target tissues, including synovium, skin, lung, choroid plexus and uveal tract, which are frequently affected in systemic immune complex diseases. No evidence of this receptor has been found in these tissues. These results suggest a mechanism different from the C3b receptor operating to localise immune complexes in these non-renal sites.