The uniformity of Xenobind covalent binding microwell plates was compared with that of passive binding plates by binding streptavidin to the plate surface, reacting the streptavidin with biotinylated alkaline phosphatase, and developing color with phenolphthalein monophosphate. Absorbance of the covalent binding plates at 570 nm was 0.95 A (CV 1.9%) compared with 0.35-0.5 A (CV 6.7-16.9%), for passive plates. No edge effect was observed with Xenobind covalent plates. Use of both plates in a model immunoassay gave a CV of 4% for the covalent binding plate and 12-15% for the passive binding plates. Moreover, the background absorbance (color development when no antigen was present) for the covalent binding plates was < 0.05 A in all cases, whereas that for the passive binding plates ranged up to 0.45 A. The high background on passive plates results from displacement of capture antibody and (or) blocking agent from the plate, thereby providing a place for detector antibody binding.