Water-in-oil emulsions containing mycobacteria constitute Freund's complete adjuvants (FCA), used to enhance immune responses. Mycobacteria are the essential constituent of FCA and various strains of these bacillus were used. The present study was to try to answer the follow question: Are there any differences between:-- water-in-oil emulsions containing mycobacteria of the bovine attenuated "BCG" strain, called Be (bovine emulsions) and--those containing mycobacteria of the human virulent "Canetti" strain, He (human emulsions)? Data presented here suggest that: 1. Bovine emulsions are active at low doses while human emulsions are only active at high doses; 2. The adjuvant activity of the bovine emulsions at low doses is attributed to both, the better stimulatary effect and the high concentration of an active component located in BCG cell envelope. Immuno-chemical studies of various extracts were conducted and the relationship between adjuvanticity and chemical structure of the active components was established.