Pertussigen, one of the biologically active proteins from Bordetella pertussis, was found highly active as an adjuvant to promote the induction of experimental allergic encephalomyelitis (EAE) in (SJL X BALB/c)F1 mice that had received at the same time an injection of mouse spinal cord (MSC) homogenized in complete Freund's adjuvant containing 4 mg of Mycobacterium tuberculosis H37RA per milliliter (CFA-H37). In this system 2 mg of MSC induced EAE, but a dose of 4 mg was more effective. As little as 250 ng of pertussigen facilitated induction of EAE, and 400 ng uniformly did so. Pertussigen was most effective when given iv from 1 day before to 5 days after administration of MSC homogenized in CFA-H37, when a uniform and severe disease was induced 11-13 days after immunization. Pertussigen given as late as 20 days after MSC-CFA-H37 still precipitated a mild form of EAE which appeared 8-12 days after the injection of pertussigen. When pertussigen was given 5 days after immunization, a chronic, nonfatal type of EAE was induced, and this persisted for the entire 74 days of observation. Histologic findings in the brain and spinal cord 15 days after sensitization in mice which received pertussigen and developed EAE showed perivascular infiltrates consisting mainly of mononuclear cells.