It was the purpose of this in vitro investigation to evaluate the effectiveness of several brands of sealed, reusable prophylaxis angles to keep internal materials within the internal portions of the head of the prophylaxis angle, and not allowing contaminates to leak out. Three brands of sealed, reusable dental prophylaxis angles were autoclaved and then taken apart under a biocontainment flow hood. Testing conditions were designed to prevent a "worst case scenario" by inoculating dental prophylaxis angles with 10(6) of the heat resistant spores of Bacillus stearothermophilus and 20% bovine serum albumin to simulate the presence of human serum. The concentration of Bacillus stearothermophilus spores was verified before testing procedures were initiated. The internal portions of the sterile prophylaxis angles were inoculated with a 1:1 mixture of the Bacillus stearothermophilus spores and bovine serum albumin, at a concentration of 1.15 x 10(6) spores/inoculation. The prophylaxis angles were reassembled under sterile conditions, and a sterile rubber cup was inserted into each of the prophylaxis angles. The prophylaxis angles were attached to a sterile dental handpiece and then submerged in a 50 ml tube containing sterile trypticase soy broth and run at 3000 rpm for 30 seconds. The tube of medium was incubated for 7 days. No growth of Bacillus stearothermophilus spores could be cultured from one of the brands of prophylaxis angles at any time during the incubation period. The other two brands of prophylaxis angles did produce some leakage of the Bacillus stearothermophilus spores.