20 Reference strains of Legionella species, isolated from human, were classified according to their ability to grow within thioglycolate-induced peritoneal macrophages of mice and guinea pigs. inbred and congenic mice were used to study the effect of the natural resistance genes Lgn1 and Bcg that are expressed phenotypically in the mouse macrophages. The Lgn1 gene controlled the intracellular growth of Legionella pneumophila Philadelphia-1 and Legionella jordanis GIFU 12657, but the Bcg gene did not affect the intracellular growth of any organism examined. Based on these results and the growth ability in guinea pig macrophages, the 20 reference strains were divided into four groups. This grouping will help us to understand a variety of modes of interaction between Legionella species and macrophages. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.