The inhibitory factor of less than 3 kDa is heat-sensitive. Size fractions of PDC supernatant were incubated at 50, 75, or 100°C for 20 minutes, followed by incubation with LuSIV cells and HIV-1 for 24 hours. RLU, relative light units. Error bars represent standard deviations. *P < .05; **P < .01 compared with control (ie, HIV only).