The following effects of Aspergillus terreus alcoholic extract are investigated: antiblastic and antiviral effect, and the ability of modification of interferon production and lymphocyte blastic transformation. In particular the extract showed more evident antiproliferative effect on transformed or EBV immortalised cells and lower effect on normal cells. Moreover doses from 12.5 +/- micrograms/ml depressed significatively the in vitro interferon production and lymphocyte blastic transformation induced by PHA on human cells. Mengo and Semliki Forest viruses replication was reduced, although temporarily, in the presence of 50 +/- micrograms/ml of the extract. Eventually the study of some fractions separated on chromatographic column demonstrated the contemporary presence of fractions able to inhibit or stimulate K-562 erythroleukemic cells replication.