Tumour necrosis factor (TNF) was first described as a factor in the serum of mice injected with tubercle bacilli (BCG) and several days later with lipopolysaccharide (LPS). The gene encoding TNF has recently been cloned and pure recombinant human TNF is now available. TNF is known for its in vivo antitumour effect and in vitro cytotoxicity on certain transformed cell lines. Similarities in amino acid sequence and biological activity to lymphotoxin and cachectin have been reported, and very recently a growth-factor-like activity on diploid fibroblasts was observed. There is no similarity between these proteins and interferons (IFNs), which are also induced during in vivo induction of TNF. Here we describe the antiviral activity of pure recombinant human TNF in a typical in vitro antiviral assay which we discovered while investigating the possible role of TNF as an inducer of IFN.