To facilitate reproducible asbestos (1332214) analysis, information is provided to assist laboratory personnel in the interpretation of the phase contrast microscopy (PCM) method of fiber counts, specified in the NIOSH Method 7400. The origin and differences of the two sets of counting rules, designated A and B in NIOSH Method 7400, are explained and the reason for recommending the A rules when the results are to be compared with the OSHA Permissible Exposure Limit (PEL) are presented. Since underloading or overloading of filters and particulate backgrounds introduces biases in counting, fiber counts from samples outside the recommended NIOSH Physical and Chemical Analysis Method (P and CAM) 239 (100 to 1300 fibers/square millimeter and with more than 50 percent of the filter area covered with particulate) should be reported as uncountable or as probably biased. NIOSH Method 7400 does not allow differentiation of asbestos from other fiber types. If optical techniques for differential counting are used, the results indicating the differences from NIOSH Method 7400 must be reported. If qualitative analysis is needed, NIOSH Method 7402, using transmission electron microscopy is recommended for counting the PCM visible asbestos fibers.