delta 12-Prostaglandin J2 (delta 12-PGJ2), a natural dehydration product of prostaglandin D2 present in human body fluids, was shown to suppress Sendai virus replication in monkey kidney cells, at doses non-toxic to uninfected cells. Dramatic inhibition of virus production could be obtained at doses of delta 12-PGJ2 which did not inhibit cellular or viral protein synthesis, suggesting an effect on virus assembly and/or maturation. At the active concentration, delta 12-PGJ2 caused a decrease in glucosamine incorporation into the virus glycoproteins HN and F, and in at least one host cell polypeptide, while it did not affect most cellular glycoproteins and it induced the glycosylation of a 47-kDa cellular polypeptide. These effects were accompanied by the induction of heat shock protein synthesis, which was found to differ in its specificity and kinetics from induction by prostaglandin A1.