The present invention relates a novel immunological effect of one peptide from Limulus anti-LPS factor protein. The immunological effect are, (1) Induce an antiviral state in both the Hep-2 and MDBK cell lines, (2) Supernatant from human mononuclear cells stimulated with peptide Limulus anti-LPS factor (LALF) is able to induce antiviral effect on Hep-2 cell line by mean of IFN-.gamma. (3) LALF peptide is able to modulate the immune response in vitro and in vivo. The present invention can be used in therapeutic and/or prophylactic treatment regimens of humans and animals to enhance their immune responses, without stimulating the production of certain biochemical mediators (e.g., TNF-.alpha.) that can cause detrimental effects, such as fever and inflammation.