BCG has been used all over the world to immunize against tuberculosis. Nevertheless in certain areas (South India) BCG vaccines failed to show any protective efficacy. Furthermore immunosuppressive cell populations have been reported in experimental mycobacterial infection in mice. The present work reports the localization and isolation of an immunosuppressor fraction from BCG. This lipid fraction called WDB inhibited the skin reactivity of delayed-type hypersensitivity (DTH) to the test antigen CEWA (crystalline egg white albumin) in guinea pigs and depressed the production of immune antibody to SRBC (sheep red blood cells) in mice. WDB is a glycolipid with an approximate mol.wt. of 62,000. By electron microscopy, WDB was located among the BCG extracellular metabolic products (ECMP) surrounding the BCG cell wall.