Thirteen monoclonal antibodies (McAbs) showing subclass-specific and subclass-restricted reactivity to human IgG have been produced by the hybridoma technique. Their specificity has been defined by passive haemagglutination (HA) and haemagglutination-inhibition (HAI) assays using a large panel pf purified IgG paraproteins of known subclass. Their use in assaying human IgG subclasses in radial immunodiffusion (RID) and in purifying human subclasses by both positive and negative immunoabsorption chromatography is illustrated.