Majority of fly laboratory strains is infected with Wolbachia, intracellular rickettsial-type symbiotic bacteria widespread in various organisms including insects and nematodes. To make the matter worse, I found that certain antisera used for fly immunocytochemistry can recognize Wolbachia bacteria in addition to their own antigens, due to impurity in the antisera generated against the recombinant fusion proteins frequently used as antigens. Thus, combinatorial use of contaminated antisera and Wolbachia-infected flies can result in serious misinterpretations, which can be avoided by curing laboratory strains of Wolbachia. (C) 2004 Elsevier B.V. All rights reserved.