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A separate report from these laboratories (1) described the findings in investigations to measure the effect of reduction of molecular weight of rIn:rCn (poly 1:C) complexes on their physical, chemical, and biological properties. The present report describes the influence of the molecular sizes of the individual homopolymers (rI, and rCn) on their ability to form the double-stranded complex (rIn:rCn) and to induce interferon and resistance to viral infections.Materials and Methods. Sources of polyribonucleotides. Polyriboinosinic acid (rI,) , and polyribocytidylic acid (rC,) with sedimentation coefficients of 12.3 and 13.2 respectively, were purchased from the P-L Laboratories, Milwaukee, Wisconsin. The dephosphoryla ted oligo-inosinic acid trimer (Ip) zI and the dephosphorylated oligocytidylic acid pentamer (Cp) *C were obtained from Miles Laboraltories, Elkhart, Indiana, All other polynucleotides were prepared by Drs. J. L. Zabriskie and E. E. Harris of the Merck Process ResearchGroup. Treatment of polyribonucleotides. Sonic radiation of the homopolymers to reduce molecular size was carried out by procedures described earlier ( 1 ) . Ribonuclease ( RNase) degradation of rC, was carried out as follows: rC, with Sw,20 of 9.2 (mol wt = 1.9 X lo5) was dissolved in 01.15 M NaC1-01.0106 M sodium phosphate buffer solution (PBS), pH 7.0, at a concentration of 1 mg/ml and was incubated with pancreatic ribonuclease A (Sigma Chemical Co., St. Louis, Mo.) at a concentration of 0.01 or 0.1 pg/ml at 23' for varying time intervals. Samples were removed and complexed with rI, (mol wt = 1.9

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