A 96-kDa protein was found on symbiosome membranes containing bacterial endocytobionts of amoebae as detected using a monoclonal antibody as a probe. The protein was also present in the symbiotic bacteria but not on any cell component of symbiont-free amoebae. It is suggested that the 96-kDa protein might be involved in the prevention of symbiosome-lysosome fusion as a putative "fusion-preventing factor."