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Chlamydia psittaci was injected in the peritoneal cavity of different strains of inbred mice. Not all of them develop a clinical disease; some, such as C 3 H, are very susceptible and died within 6 days. C 57 BL, however, are resistant even to a very high dose. We still do not know the exact mechanism of the natural resistance. This is the reason why we are trying to analyse the role of the macrophage. This cell is particularly important in C. psittaci infection, because it harbours the obligate intracellular organisms which multiply inside the cytoplasm. Peritoneal macrophages of three different strains of mice (C 3 H, AKR and C 57 BL) were cultured following Fauve's technique [4]. 48 h after, the macrophages were inoculated with a suspension of C. psittaci. Hela 229 were used as control. 48, 78 and 92 h after inoculation, amount of inclusion in each category of macrophages and Hela cells was determined. C 57 BL macrophages cultured from mice resistant to Chlamydia psittaci allowed the in vitro multiplication of the same bacteria: 72 h after inoculation, 100% of the macrophages were infected and a large inclusion could be seen. C 3 H macrophages cultured from mice very susceptible to Chlamydia psittaci seemed less susceptible in vitro than the C 57 BL macrophages; 72 h after inoculation, 50% only of the infected macrophages showed intracytoplasmic inclusion: AKR macrophages were also susceptible, although AKR mice recover from C. psittaci infection. In conclusion, the difference of susceptibility, as displayed by several strains of inbred mice infected by C. psittaci, cannot be explained by the difference of susceptibility of their macrophages.