Assessment of the variability and bias of fiber count data, used to measure exposure to asbestos (1332214), was discussed. Detailed quality control procedures presented in NIOSH Method 7400 were reviewed. A quality assurance program utilizing recount samples and reference samples was outlined. Intracounter, intralaboratory, and interlaboratory sources of variability were considered. Count and recount data were used to assess the bias in fiber counting. A method for calculating intracounter s(r), an estimate of the relative standard deviation, was presented. A test comparing s(r) of a sample set and the s(r) historically obtained by the counter allowed rejection of fiber counts with excess variability. The limits of the test were evaluated. Intralaboratory bias and variability were assessed by comparing fiber counts of reference samples obtained by different counters in the laboratory. A method for evaluating interlaboratory s(r), involving counting 20 samples in each laboratory, was outlined. Confidence limits on the data could be calculated from the subjective component of the interlaboratory s(r). The authors recommend that a Quality Assurance Coordinator from each laboratory assess the quality of fiber count data by the methods presented.