To investigate the nature of peanut agglutinin (PNA) binding cells in various human lymphoid tissues a double marker assay was performed using fluorescent PNA and rosetting with anti-mu or anti-delta coated ox red blood cells for detection of B cells or rosetting with sheep red blood cells for detection of T cells. In human bone marrow 60.5 +/- 8.6% of the surface mu+ve (smu+ve) B cells did bind PNA whereas only a small minority of the surface delta +ve (delta +ve) B cells (5.0 +/- 4.2%) and none of the T cells were PNA+ve. In peripheral blood most of the PNA+ve cells appeared to be monocytes. Only a small proportion of the smu+ve B cells (9.7 +/- 2.7%) and none of the s delta +ve B cells or T cells did bind PNA. Contrarily in tonsils a relatively high proportion of smu+ve B cells (33.2%) of s delta +ve B cells (26.3%) and of T cells (17.2%) were PNA+ve. These results indicate that PNA binding is also a marker for immature B cells. Moreover in human bone marrow at least two populations of B cells may be distinguished, an immature population of smu+ve, s delta-ve, PNA+ve B cells and a mature populations of smu+ve, s delta +ve, PNA-ve B cells, the latter probably representing recirculating B cells. We hypothesize that the first population comprises immature B cells, that leave the bone marrow in an early stage and complete the maturation to immunocompetent B cells in peripheral lymphoid organs like tonsils.