A combination of the 69 kDa outer membrane protein and filamentous hemagglutinin (FHA), both isolated from pertussis toxin (ptx) minus mutants of B. pertussis, is protective in the mouse intracerebral challenge potency test ("Kendrick test"). A combination of the same 69 kDa protein and lymphocytosis promoting factor (LPF, pertussis toxin, PTX) is approximately 15 times less effective. The data show that the 69 kDa protein in tandem with FHA is the most relevant combination for mouse protection, and may be a more suitable acellular pertussis vaccine candidate than the LPF/FHA combination which has never been satisfactorily protective in the mouse test. However, the preparation of the 69 kDa protein of acceptable quality is still difficult. Attempts were made at screening the most suitable batches of the preparation by exploiting some recently discovered properties of the 69 kDa protein, i.e. its chromatofocusing pattern and lymphocyte affinity. Although both these properties may differentiate 69 kDa preparations, none of them was able to replace the mouse intracerebral challenge potency test for the final quality assessment.