Malignant lymphomas have come to be recognized as neoplasms of the immune system. These lymphoproliferative disorders demonstrate surface and cytoplasmic antigenic phenotypes that reflect qualitative and quantitative alterations or aberrant expression of genetic material. Traditionally, cell suspension studies have been used for phenotypic analysis. Alternative immunohistologic methods can be used to profile immunophenotypes in situ. Most lymphoproliferative disorders can be readily classified as T or B cell malignancies, and criteria have been evolved to differentiate neoplastic from reactive/physiologic expansions of lymphoid clones. However, antigenic phenotypic expression does not always correspond to known immunophenotypes of subsets of T or B cells and probably reflects the complexity of neoplastic transformation. Currently, frozen tissue sections, preferably in combination with cell suspension analysis using cytocentrifuge preparations and/or flow cytometry, can provide information to phenotype lymphomas classified by the International Formulation or other nomenclature. Their continued utility depends on development of and adherence to strict quality assurance programs.