Lymphoid cells in human bone marrow are either assembled focally or occur in a diffuse, loosely scattered infiltrate. While the focal lesions are easily detected, the lymphoid cells of the diffuse infiltrate are hardly recognizable with conventional stains. Quantitative immunohistological analysis of 103 trephine biopsies, including cases with reactive disorders (e.g. myeloid hyperplasia, aplastic anaemia) and neoplastic processes (e.g. myeloproliferative disorders, B-cell non-Hodgkin's lymphomas) and some specimens with normal architecture yielded the following results: (1) Various antibodies recognizing B cells (L26, 4KB5, MB1, Ki-B3), T cells (UCHL1, MT1) and NK cells (Leu-7) are effective in paraffin-embedded bone marrow sections, thus enabling analysis of the in situ distribution of normal lymphocyte subsets and subtyping of lymphomatous infiltrates. (2) The lymphocytes of the diffuse infiltrate constituted about 1-5% of all nucleated cells in normal bone marrow. (3) In the diffuse infiltrate, T lymphocytes were regularly observed in higher numbers than B cells, and Leu-7+ cells were rare or virtually absent, irrespective of the diagnosis. (4) The focally assembled lymphoid cells were mainly B lymphocytes, but many T cells were always intermingled. This was true for both reactive follicles and neoplastic lymphomatous infiltrates, which generally cannot be differentiated on the basis of immunohistological findings alone.