An automated endoscope sterilizing machine, the Steris System 1 Processor, was tested for bactericidal and sporicidal efficacy. The disinfectant, peracetic acid, was diluted to 0.2% within an enclosed system. The exposure time to the disinfectant was 12 min and the overall cycle time ranged from 25-38 min, mean 29 min. Preliminary suspension tests, with and without yeast or serum, showed a log10 reduction of > 5 with Pseudomonas aeruginosa, Staphylococcus aureus and Bacillus subtilis in 5 min with 0.2% peracetic acid. After a routine cycle in the machine, endoscopes contaminated with the same organisms showed no growth. Two of 24 spore strips, containing 10(6) B. subtilis showed a small number of survivors (less than 10 per strip). No significant damage to the endoscope was observed although the number of cycles tested was small (i.e. 31). The advantage of the system is that staff are not directly exposed to the agent, but the costs per cycle are higher than glutaraldehyde, since peracetic acid is not renewed. Unlike other automated processors the Steris machine has no cleaning cycle.