Techniques were developed for the preparation of frozen sections from undecalcified bone marrow biopsy cores and bone marrow aspirates. Selected indirect antibody and biotin-avidin detection systems, employing immunoperoxidase or immunofluorescent labels, were studied to determine which were best suited for bone marrow frozen section immunohistopathology. Methods employing murine hybridoma monoclonal antibodies illustrate the immunophenotyping of representative lymphoid neoplasms, involving bone marrow. The results of immunohistologic staining were comparable to those of fluorescence-activated cell sorter (FACS) analysis of cell suspensions. The advantages and limitations of immunohistologic techniques are discussed as they relate to immunophenotypic studies previously feasible only with bone marrow cell suspensions. Frozen section immunohistologic techniques serve as useful adjuncts to the conventional evaluation of bone marrow aspirates and biopsies.