Alkylating agents, cyclophosphamide (CY) and the related compound mechlorethamine (NM), significantly increase in vivo the blood level of IL-6 but not of IL-1. Since in vitro CY is inactive we have used in our experiments NM, a compound structurally and functionally related to phosphoramide mustard, the natural biologically active metabolite of CY. Thioglycollate or oil-induced peritonal macrophages (Mf) of four different mouse strains treated with NM produce significantly more IL-6 than the non-treated cells. In contrast, under these conditions, the production of IL-1, TNF alpha and NO/NO2 radicals is not affected. The NM-induced elevated production of IL-6 by Mf could not be further increased by the treatment of cells with LPS, which may suggest that both agonists stimulate the same signalling pathway. The antineoplastic activity of the alkylating agents like CY or NM is usually attributed to the interstrand cross-linking of the DNA of the dividing cells. Our experiments can be tentatively interpreted as the induced overplus of IL-6 can also contribute to the stimulation of the cytotoxic NK-cells.