A series of 3' and 4'-substituted avarone derivatives were synthesized and tested in culture systems as antitumour and antiviral agents in comparison to avarol and avarone. 3'-Alkylamino derivatives showed potent cytostatic activities against murine L1210 and human B (Raji) and T (C8166, H9) lymphoblast cells (ID50 range 1.7-3.7 μM). Avarol and avarone were six times less active. While none of the derivatives showed anti-human immunodeficiency virus (HIV) activity superior to that of the parent compounds, most of them, avarol and avarone included, were potent and selective inhibitors of poliovirus multiplication.