Peanut (Arachis hypogaea) lectin (PNL) has been shown to agglutinate the 90% of cells from murine thymus which are supposed to be immature cortical thymocytes. Further studies on the numbers of thymocytes binding fluorescein isothiocyanate conjugated PNL (FITC-PNL) confirmed the large proportion of PNL binding cells. In other organs such as bone marrow, spleen and peripheral lymph nodes, smaller proportions of PNL positive cells have been recorded. PNL-positive cells outside the thymus have been reported to be either Thy 1-positive or null cells. It has also been suggested that PNL binding may be a marker for immaturity not only in relation to T lymphocytes but also amongst haematopoietic stem cells. Thus PNL binding as an aspect of lymphocyte differentiation is a matter of considerable interest. The current study describes the distribution of horseradish peroxidase-conjugated PNL (HRP-PNL) on frozen sections of mouse lymphoid organs. It seems that PNL binds to cells in germinal centres but not to those in some other areas containing activated lymphocytes. There is good correlation between the presence of PNL-binding germinal centres in frozen sections of lymphoid organs and the number of PNL-binding cells counted in cell suspensions from the same organs.