Mutagenic activity of aflatoxin B1 (AFB1) was followed in the Ames Salmonella/microsome test using liver S-9 from uninfected and Fasciola hepatica infected mice. The S-9 preparations from parasite-infected animals were capable of inducing significantly greater AFB1 mutagenic activity to strain TA100 than S-9 preparations from either control, uninfected mice or Aroclor 1254-induced mice. The differences in activity between infected and uninfected mice were consistent between sex and strains of mice studied. The observed increase in biological activity was apparently due to an induction phenomenon in the host tissues rather than due to metabolism of AFB1 by F. hepatica enzymes.