Background: some natural and synthetic compounds can prevent, suppress, or reverse the progression of virus infection. Natural products have proven to be the most effective in terms of their ability to act as an antiviral. In the present study, the antiviral potentiality of the bioactive compounds derived from aqueous extract of two Egyptian marine seaweed species (Cystoseiramyrica and Ulva lactuca)were assessed on different viruses. Materials and methods:these two species were collected from Hurghada at the Red Sea and Al-Agami area in Alexandria Mediterranean Sea, Egypt.The assay of cytotoxicity and antiviral activity by MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenlytetrezolium bromide] and by neutralization methods were conducted. Results:these two species have a bioactive compound affected hepatitis A virus (HAV-H₁₀), Coxsackie B₄ virus,herpes simplex virus types-1 (HSV-1) and type2 (HSV-2), grow and cytopathic effect (CPE) in Vero cells.