The research report represents important information on whether the current technique of in-facepiece sampling provides representative samples of airborne contaminants from within the facepiece cavity of negative pressure half and full-facepiece respirators. The adequacy of in-facepiece sampling methodology is important for assessments, mandated by the Toxic Substances Control Act, and conducted by EPA's Office of Toxic substances (OTS), of the level of respiratory protection that can be provided to workers engaged in the proposed manufacture of new chemical substances. The precision and bias associated with five different methods of sampling for inboard penetration through three areas on the face seal of full and half facepiece negative pressure respirators were determined. The sampling procedures identified and evaluated in the study were: (1) a continuous, low sampling rate, flush, mid nose-mouth probing (CLF) procedure; (2) a continuous, high sampling rate, deep front-of-mouth probing (CHD) procedure; (3) a pulse exhalation, high sampling rate, deep front-of-mouth probing (PED) procedure; (4) an exhalation valve discharge (EVD) procedure; and (5) a pulsed inhalation, high sampling rate, deep front-of-mouth probing (PID) procedure. The CLF procedure is used to represent current in-facepiece sampling practice in the U.S. Evaluations were conducted on nine models of full facepiece respirators not equipped with nose cups. The bias observed with each method was found to be affected, to some extent, by the location of the face seal penetration.