Complement activation, cell surface-receptor binding, blocking activity, and possibly placental transfer are among the biologically important functional differences that have been detected between the four human IgG subclasses by use of polyclonal antisera. In 1985, a IUIS/WHO panel of immunologists, using eight immunological methods, documented the specificity of select monoclonal antibodies for the IgG subclasses. Clinical assays have been developed involving these monoclonal antibodies that allow quantification of the concentration of IgG subclass protein and distribution of the IgG subclass antibodies in human immune responses. This review addresses issues of concern to investigators who are evaluating and (or) developing quantitative human IgG subclass assays in the clinical laboratory. Unique physical (structural) and biological (functional) properties of human IgG subclasses are summarized, with a focus on aspects pertinent to their clinical importance and in vitro quantification. The HP-series monoclonal antibodies with documented specificity are examined within the context of their application to several immunological methods. I describe unique technical aspects of total and antigen-specific IgG-subclass immunoassays involving these monoclonal antibodies. Finally, this report outlines clinical applications and indications for IgG-subclass measurements in the study of human health and disease.