The antiviral action of cyclopentenone prostaglandins (PGs) is generally associated with alterations in the synthesis and/or maturation of specific virus proteins. In particular, inhibition of Sendai virus (SV) replication in African green monkey kidney cells by PGA1 has been shown to be a cell-mediated event, due to alterations in SV protein glycosylation and accompanied by the induction of a cellular polypeptide of Mr 74K. In this report we identify this protein as a heat shock protein (HSP) related to the major 70K HSP group (HSP70). Induction of HSP70 synthesis by PGA1 was found to be dose-dependent, and an accumulation of HSP70 comparable to that occurring after heat shock could be obtained at concentrations of PGA1 that did not inhibit macromolecular synthesis in uninfected cells, but caused a dramatic block of virus replication in SV-infected cells. Induction of HSP70 by PGA1 occurred at the transcriptional level and was not affected by SV infection. HSP70 synthesis was evident between 2 and 3 h after PGA1 treatment, maximal at 12 h and went back to control levels by 26 h after the addition of PGA1, thus preceding virus protein synthesis. Finally, of several PGs tested, only those which possess antiviral activity induced the synthesis of HSP70. These results, together with the observation that suppression of HSP70 synthesis by actinomycin D also abolishes the PGA1-induced alteration of SV glycoproteins, suggest that HSP70 could play a role in the block of virus replication by cyclopentenone PGs.