According to Pharmacopoea Nordica, steam autoclaves should be regularly monitored by a specific Swedish preparation of Bacillus stearothermophilus spores. If another biological indicator (BI) is used for such a control, it should first be calibrated against the Swedish BI (SBI) and the two BIs should be equally thermoresistant. Attest No. 1262 BI (ABI) has previously been shown to be more thermoresistant than the SBI at 134 degrees C, saturated steam. The purpose of the present study was to compare the thermoresistance of the SBI and the ABI at 121 degrees C, saturated steam and prevacuum. Seven hundred and twenty units of each BI were heat-exposed in an Emmer 760 litre prevacuum, pressure-pulsing steam autoclave. After prevacuum with steam injection (manual or automatic preconditioning), the following incremental heat exposure times were used in triplicate (20 simultaneously tested units of each BI in each cycle) according to a randomized scheme: 5, 6 1/2, 8, 9 1/2, 11, 12 1/2, 14 and 15 min. The intra-chamber pressure and temperature were continuously monitored throughout the test and equilibration cycles. The heat-exposed BI units were cultivated and read as recommended by the manufacturers. SBI and ABI showed a survival-time of 8 min and 11 min respectively, and a kill-time between 14 min and 15 min for both BIs. Thus, the ABI had the narrower survival-kill window. Probit analysis testing of the results showed that the difference in thermoresistance, at 121 degrees C, saturated steam and prevacuum between Attest No. 1262 BI and the Swedish BI mentioned in Pharmacopea Nordica was not statistically significant.