Egg-washing with isotonic solution has been used to improve the artificial fertilization in fresh-water salmon aquaculture.Usually, unfertilized eggs are rinsed twice or three times with isotonic solution and showered before fertilization. We investigated the effect of egg-washing using rainbow trout eggs on elimination of some causative agents of cold water disease (Flavobacterium psychrophilum), flunculosis (Aeromonas salmonicida), infectious hematopoietic necrosis (IHNV) and Oncorhynchus masou virus disease (OMV), quantitatively and on prevention of intra-ovum infection with F. psychrophilum. The number of live bacterial cells or virus infectivity titers which were eliminated by egg-washing, was 10³CFU/ml in F. psychrophilum, 10⁵CFU/ml in A. salmonicida, or 10⁴TCID₅₀/ml in both of IHNV and OMV. In shower washing, the eliminated extent of F. psychrophilum or A. salmonicida cells increased with increasing isotonic solution volume. The ratio of intra-ovum infection with F. psychrophilum was clearly less in rinsed eggs than in non-treated eggs, and result of detection from showered eggs was negative. These results suggest that egg-washing with isotonic solution is very effective to eliminate the bacteria or viruses on egg surface and is able to decrease the risk of intra-ovum infection.