The thermal denaturation of poliovirus virions and procapsids in the 42 to 48 degrees C range was studied using N- and H-specific monoclonal antibodies. The half-life of the N antigen of Mahoney and Sabin 1 virions was extended 50- to 250-fold by either 10 microM WIN 51711 or R 78206. The minimum concentrations required for full stabilization at 46 degrees C (1.0 microM for WIN 51711, 0.2 microM for R 78206) were independent of the strain or serotype of the virus; 30 to 60 molecules of stabilizer per virion were required for full protection. R 78206 was the most efficient stabilizer of Mahoney procapsids; the half-life of the N-specific epitopes of these particles at 44 degrees C was extended from less than 1 min to 1 day.