Eleven fish cell lines were compared for susceptibility to red sea bream iridovirus (RSIV). Cytopathic effect was produced in BF-2, CHSE-214, FHM, JSKG, KRE-3, RTG-2 and YTF cell lines among the cell lines examined. The titers of the virus on BF-2 and KRE-3 were higher than those on the other cell lines. The pretreatment of BF-2 and KRE-3 cells with polyethylene glycol (PEG) or addition of PEG to the sample increased in the virus titers. RSIV replicated at 15 degrees C, 20 degrees C and 30 degrees C but not at 37 degrees C. The suitable temperature for viral growth was 20 degrees C or 25 degres C. The virus was sensitive to acid (pH 3), chloroform and ether and unstable to heat but stable to ultrasonic treatment and repeated freezing and thawing. Treatment with 5-iodo-2-deoxyuridine (IUdR) reduced the titers of the virus.