Abstract: A diagnostic reagent holder specifically adapted for the detection of an unknown quantity of a biologically derived sample suitably by radioimmunoassay, fluorometric detection or by spectrophotometry. The holder comprises an elongate shaft with a support surface adjacent one end of the shaft. Diagnostic reagent is covalently attached to a self-supporting film such as a disc and firmly secured to the support. The diagnostic reagent, e.g., antibody, is stirred during incubation in contact with the sample to be detected, e.g., antigen, in solution. In a "sandwich" technique labelled substance (e.g., fluorochrome-labelled antibody) is reacted with the sample on the film followed by thorough washing. Then, the holder is slid into a novel viewing housing which precisely positions the film at a window for viewing as with a fluorometer.