Protection factors for one model of closed-circuit, pressure-demand, self-contained breathing apparatus were determined for eight volunteers while clean-shaven and at approximately two-week intervals during an eight-week, beard-growth period. Measurement of facepiece leakage was patterned after the DOP quantitative fit test method outlined in ANSI Z88.2-1980. Comparison of results indicated that beard growth had a profound adverse effect on the amount of protection provided.