The proliferation of characteristic cells in expanded sinuses and in the perivenular area of the paracortex of lymph nodes with patients with toxoplasmosis, and homosexual and drug abusers with lymphadenopathy is referred to as 'immature sinus histiocytosis' (ISH). In a study of nine cases we have shown that these cells are monocytoid B-cells with an immunological phenotype differing from that of other node-based B-cells. The function of these cells is not known.