Hepatitis A virus (HAV) growing in human diploid lung fibroblast (MRC5) monolayers can either interfere with or enhance the cytopathic effect of Newcastle Disease virus (NDV) challenge. Enhancement of NDV occurred if HAV-infected monolayers were challenged with a low multiplicity of infection of NDV and incubated at 35 degrees C. Interference occurred if HAV-infected monolayers were given a high NDV multiplicity of infection and incubated at 32 degrees C. These phenomena were applied to assays for quantifying HAV and may be useful in providing new insights into viral interference and enhancement.