The lymphocyte proliferation assay is used as an in vitro surrogate, similar to the in vivo delayed-type hypersensitivity assay, to assess the overall quality and character of the cellular arm of the immune response. The lymphocyte proliferation assay is used to assess congenital immune deficiencies, transient immune compromised states, and more recently, the progressive immune deficiency of HIV infection (see Note 1) (1-19).