Abstract Poly(I) · poly(C) molecules were trapped with reconstituted Sendai virus envelopes when added to the reconstitution system. A quantitative estimation indicated that about 10% of the added poly(I) · poly(C) remained associated with the fusogenic viral envelopes. About 50% of the associated poly(I) · poly(C) were found to be RNAase A resistant, enclosed within the viral envelopes. Incubation of loaded viral envelopes with HeLa or L-cells resulted in strong inhibition of protein synthesis, indicating fusion-mediated microinjection of the enclosed poly(I) · poly(C). Introduction of poly(I) · poly(C) into cultured cells by the use of reconstituted Sendai virus envelopes was as efficient as the introduction of these polynucleotides using the calcium phosphate coprecipitation technique. The inhibition of protein synthesis in L-cells but not in HeLa cells was dependent upon pretreatment with interferon. Incubation of poly(I) · poly(C)-loaded viral envelopes with interferon-treated variant cells of the NIH 3T3 line, which possess a very low amount of RNAase L, resulted in only 25% inhibition of protein synthesis, compared to 85% inhibition observed in L-cells.