Association of the ribosome-inactivating proteins (RIPs): pokeweed antiviral protein (PAP), gelonin, Momordica charantia inhibitor (MCI), with reconstituted Sendai virus envelopes (RSVE) was obtained without detectable loss of activities either of RIPs or of viral envelope glycoproteins. RIPs are inactive towards intact cells, but, once encapsulated in RSVE, they become cytotoxic. The concentration of RSVE-associated PAP, which causes 50% inhibition of protein synthesis by Friend erythroleukemic cells, is 0.5 ng/ml. Substances capable to inhibit the viral activities block the acquired cytotoxicity of RIPs associated to RSVE.