Variables believed to affect the amount of interferon (IF) produced in bovine embryonic lung cell cultures by polyriboinosinic-polyribocytidylic acid (poly (rI:rC)) were investigated. A concentration of 100 micron of poly (rI:rC)/ml consistently induced substantial amounts of IF. Bovine viral diarrhea virus infection of cultures did not interfere with induction of IF by poly (rI:rC). Neither the age of cells nor the passage level of the cultures seemed to affect the amount of IF produced. Priming of cultures with large concentrations of IF before treatment with poly (rI:rC) was more effective than priming with small concentrations of IF. Furthermore, priming for 24 hours was more effective than was priming for 12 hours. The use of 2% bovine fetal serum in the medium, subsequent to treatment with poly (rI:rC) markely enhanced IF production. Practically all IF was produced within a few hours after treatment with poly (rI:rC).