A simple method is described for rapid determination of immunoglobulin class and subclass in an assortment of samples based on the slide immunoenzymatic assay (SIA-Ig). Each circle on a multi-circle glass slide is coated with anti-immunoglobulin class or subclass antibody. For each isotype to be assayed a circle is coated with its specific anti-isotype. The coated circles are incubated with sample containing the immunoglobulin(s) of unknown isotype and are then washed. The slide is then incubated with enzyme-labeled anti-immunoglobulin and washed again. Finally, enzyme substrate is deposited onto the circles. Color appears within a few minutes only on the circle where the unknown was bound specifically by its corresponding anti-isotype antibody. The method reveals correctly the isotype of the constituents of complex mixtures, such as serum, as well as that of the only component of samples containing a single molecular species of immunoglobulin (e.g., monoclonal antibodies). The method is simple, reliable, gives results in less than 1 h and is adequately sensitive for a wide range of practical applications.