The recombinant bacillus Calmette-Guerin (rBCG) secretion system utilizing an extracellular alpha antigen of Mycobacterium kansasii (alpha-K) was characterized biochemically and immunologically. The human immunodeficiency virus type1 (HIV-1)p17(gag) B cell epitope fused to alpha-K was secreted in extremely large amounts. At least three mice out of seven inoculated with rBCG generated high titres of antibody to the epitope. The long-lasting antibody production persisted more than 14 months.