Paper strips inoculated with spores of Bacillus stearothermophilus ATCC 7953 were conventionally dried (lot 1) and lyophilized (lot 2); stored in defined environments of 32 and 86% relative humidity at 10, 25 and 33 C for 210 d; and submitted to moist heat treatments at 121 C. A significant decrease in thermal resistance from initial starting levels was found for lyophilized bioindicators stored at 86% relative humidity. The respective average D-121degreesC values were 1.55 +/- 0.05 and 1.37 +/- 0.10 min for lyophilized bioindicators stored at 32 and 86% relative humidity; and 1.65 +/- 0.15min and 1.57 +/- 0.11 min for dried bioindicators stored in the same environments.