Abstract: Animal infection by injecting intraperitoneally Legionella pneumophila (Lp) into guinea pigs demonstrated that the optimal time for rediscovery of LP was 4 to 5 days after infection and the spleen and lungs of infected animal were the best choice of organs for reisolation of Lp. We also found that concentrating water samples by aluminium potassium sulfate resulted more Legionella colonies than those by centrifugation. The proper procedure of isolating Legionella from environmental water was thus found out: aluminium potassium sulfate-guinea pig-BCYE_α medium. In order to practise this procedure. 29 lots of water sample were collected from air-conditioning system at two hotels in Beijing and treated with above procedure, The success of isolating s strains of Lp showed the feasibility of this procedure.