Inbred strains of mice display different susceptibilities to experimental induction of reactive amyloidosis. CBA/J, C57BL/6, and ICR are among the most susceptible strains, while CE/J mice appear to be totally resistant. In contrast to amyloidogenic strains which express two major acute phase serum amyloid A proteins (SAA1 and SAA2), CE/J mice produce only a single isoform, designated SAA2.2. Studies indicate that CE/J x CBA/J hybrid mice expressing both SAA2.2 and SAA1.1/SAA2.1 are amyloid resistant, and this has led to the hypothesis that SAA2.2 may protect mice against amyloid formation even in the presence of fibrillogenic SAA1.1. We have tested this hypothesis in mice derived from CE/J and C57BL/6 strains. CE/J mice were mated with C57BL/6 mice to produce F1 hybrids. Congenic mice were then produced by backcrossing each successive generation to C57BL/6 mice. Representative mice from F2 and F3 generations were analyzed to determine SAA genotype and susceptibility to amyloid induction by repeated casein injections. All F2 and F3 mice examined, including those which carried the SAA2.2 gene, developed extensive splenic AA amyloid. Expression of SAA2.2 in mice testing positive for the SAA2.2 gene was confirmed by sequence analysis of HDL-associated SAA proteins. These results demonstrate that the presence of SAA2.2 is not sufficient to protect CE/J x C57BL/6 hybrid mice from amyloid development. This is consistent with our observation that macrophage cultures, derived from C57BL/6, CBA/J, or CE/J mice, undergo amyloid deposition when treated with SAA1.1 alone or with equal amounts of SAA1.1 and SAA2.2, but show no deposition when treated solely with SAA2.2. We conclude from these studies that while SAA2.2 is non-fibrillogenic, its physical presence is not sufficient for protection against amyloid formation.