Abstract: A semi-automated biological sample analyzer and subsystems are provided to simultaneously perform a plurality of enzyme immuno assays for human IgE class antibodies specific to a panel of preselected allergens in each of a plurality of biological samples. A carousel is provided to position and hold a plurality of reaction cartridges. Each reaction cartridge includes a plurality of isolated test sites formed in a two dimensional array in a solid phase binding layer contained within a reaction well which is adapted to contain a biological sample to be assayed. The carousel and cartridges contain structures which cooperate to precisely position the cartridges in each of three separate dimensions so that each cartridge is positioned uniformly. An optical reader operating on a principle of diffuse reflectance is provided to read the results of the assays from each test site of each cartridge.