In a study of genes expressed differentially in the freshwater crayfish Pacifastacus leniusculus infected experimentally with the white spot syndrome virus (WSSV), one protein, known as antilipopolysaccharide factor (ALF), was chosen, among those whose transcript levels increased upon viral infection, for further studies. ALF RNA interference (RNAi) experiments in whole animals and in cell cultures indicated that ALF can protect against WSSV infection, since knockdown of ALF by RNAi specifically resulted in higher rates of viral propagation. In a cell culture of hematopoietic tissue (Hpt) from P. leniusculus, quantitative PCR showed that knockdown of ALF by RNAi resulted into WSSV levels that were about 10-fold higher than those treated with control double-stranded RNA (dsRNA). In addition, RNAi experiments with other crayfish genes that had been found to be up-regulated by a WSSV infection did not result in any changes of viral loads. Thus, the cell culture does not respond to dsRNA in a similar manner, as shown earlier for dsRNA injected into shrimp, which gave a higher degree of resistance to WSSV infection. If ALF transcription in whole animals was stimulated by the administration of UV-treated WSSV, a partial protection against a subsequent challenge with the active virus was conferred to the host. This is the first crustacean gene product identified with the capacity to interfere with replication of this important pathogen.