EM showed that commercial poly I:C consisted of molecules varying in length from less than 0.05 nm to more than 5 nm. To clarify the relationship between its molecular size and [mouse] interferon-inducing activity, poly I:C was sonicated or fractionated by sucrose density gradient centrifugation. The molecular length distribution and interferon-inducing activity of each preparation was determined in vivo and in vitro. Poly I:C molecules 0.1-0.3 nm long were the most effective for interferon induction. Sonication of poly I:C reduced its molecular length and the interferon-inducing activity, the degree of reduction varying in different fractions. The interferon-inducing activity of poly I:C of 0.1-0.3 nm obtained by sucrose density gradient centrifugation was higher than that of poly I:C of corresponding length prepared by sonication.