The potential antiviral effects of the culture filtrates (CF) from Serratia marcescens strain Gsm01 against yellow strain of Cucumber mosaic virus (CMV-Y) were investigated. The culture filtrate of S. marcescens strain Gsm01 applied on Chenopodium amaranticolor showed high inhibitory activity, likewise no necrosis appeared when applied on the tobacco plants 2 days before CMVY inoculation. When plants were challenge inoculated with CMV-Y for eighteen days, the disease incidence in plants with culture filtrate of S. marcescens Gsm01 did not exceed 59%, whereas 100% of control plants were severely infected. The results of double antibody sandwich- enzyme linked immunosorbent assay (DAS-ELISA), reverse transcriptase polymerase chain reaction (RTPCR), dot blotting, and western blotting showed that culture filtrate treatment highly affected the accumulation of CMV-Y or its CP protein gene in the treated plant leaves. It was also observed that the culture filtrate had no RNase activity on genomic RNAs of CMV-Y, suggesting that culture filtrate may not contain ribosome inactivating proteins (RIPs) or proteins with RNase activity. These data shows that culture filtrate of S. marcescens strain Gsm01 seems to be a promising source of antiviral substance for the practical use.