The data reported here demonstrate that a preparation extracted from nonpathogenic mycobacteria such as Mycobacterium smegmatis and hereafter referred to as interphase material protected mice against Ehrlich ascitic carcinoma, L-1210 leukemia, and another syngeneic lymphoid leukemia. Furthermore, mice treated by this preparation were much less susceptible to endotoxins than when stimulated by BCG (bacillus Calmette-Guerin) or M. smegmatis cells. Moreover, guinea pigs treated by interphase material administered in Freund's incomplete adjuvant showed an increased immune response, yet their sensitivity to tuberculin was much weaker than that of controls sensitized with Freund's complete adjuvant.Finally, resistance to Columbia SK virus infection could be demonstrated when interphase material was administered to mice prior to virus challenge.It is now well established that mycobacteria have the ability to stimulate the immunity of the host against tumors (1-2). However, these organisms have also the capacity for rendering mice very susceptible to endotoxins (13) and for inducing delayed hypersensitivity to tuberculin. Our previous studies have shown that delipidated cells as well as purified cell walls of Mycobacterium smegmatis or M. kansasii had greater antitumor activity than killed cells of bacillus Calmette-Guerin (BCG), although they were devoid of several of the noxious effects of whole mycobacterial cells (14).In the following study various biological activities of a preparation extracted from nonpathogenic mycobacteria and hereafter referred to as interphase material (IPM) the press with cooling. The resulting homogenate was centrifuged for 90 min at 5000 X g, yielding a multiphase system consisting of a lower cell debris layer, a lower opaque aqueous layer and an upper hexane layer containing a sedimented gelatinous phase. The aqueous phase was withdrawn from the system by aspiration and discarded. The hexane phase containing the gelatinous sediment was decanted and centrifuged at 10,000 X g for 1 hr. The gelatinous material was separated from the hexane by decantation, suspended in distilled water, and the suspension was freed from residual hexane by heating to about 380 under reduced pressure. The aqueous suspension was freeze-dried to yield 29.0 g of a fluffy, light colored amorphous solid hereafter referred to as M. smegmatis interphase material (IPM). A similar procedure was used for obtaining interphase material from M. phlei and M. kansasii.M. smegmatis IPM is insoluble in neutral aqueous systems and only partly soluble in organic solvents. It is characterized by the following elemental analysis: C, 51-57%; H, 7.5-8.7%; N, 5.8-6.5%; P, 0.5-1.2% and by the following composition: neutral carbohydrates (arabinose, galactose, glucose), 21-23%; amino sugars, 0.8-1.8%; amino acids (as peptides or proteins), 24-28%; free lipids, 40-43%. These data as well as the presence of a,a'-diaminopimelic acid and approximately equimolar amounts of glucosamine and muramic acid indicate that IPM is essentially...