The effect of 1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide (ribavirin) on tolerance induction with rabbit gamma globulin (RGG) was investigated. The treatment with ribavirin, initiated on the 4th and 17th day of life, did not reduce the immune response significantly, but provided a much more profound level of tolerance than could be achieved in animals not treated with ribavirin. When treatment with ribavirin was initiated at the age of 50 or 65 days, the response of immunized animals was reduced and the response of tolerized-immunized animals was increased. The mechanism, underlying the drug action, was discussed in terms of an effect or ribavirin on the balance between help and suppression; the possibility that a virus may be involved in age-dependent changes and that ribavirin prevents it proliferation, was considered.