- An aqueous extract of propolis was found to significantly inhibit NIH/3T3 cell malignant transformation by Moloney murine sarcoma virus (MuSV-124). The inhibitory effect of propolis extract was most effective when it was added 2 h before infection or at the time of infection. The continuous presence of propolis extract in the culture medium was essential for full prevention of malignant cell transformation. When treatment with propolis extract was terminated, five to ten days post-infection, there was a significant recovery in cell transformation. These results suggest that propolis extract inhibits a late step after provirus integration into the host genome. Addition of propolis extract after infection with MuSV significantly inhibited cell transformation. The inhibitory effect of propolis appeared to be the result of the inhibition of primary – not secondary – infections, since MuSV-124 yields a virus-nonproducing infection.