The multirule quality control procedure for interpreting Bull's moving averages, proposed by Levy and colleagues (Am J Clin Pathol 1986; 85: 719-721.), has been evaluated by computer simulation with the use of the approach of Cembrowski and Westgard (Am J Clin Pathol 1985; 83: 337-345.). With this procedure, a batch of patient specimens is rejected if either of two criteria are satisfied: (1) the Bull's mean of one of the red blood cell indices is outside its 3% limits, or (2) the average of three consecutive Bull's means is outside its 2% limits. Power function curves were used to summarize the performance of the multirule approach and demonstrated error-detection capabilities that are superior to the more common implementation of Bull's algorithm using 3% limits for single Bull's means. The increased error detection achieved by the multirule procedure allows shifts in hemoglobin and mean corpuscular volume to be more readily detected but also results in the detection of small shifts in red blood cell count. A modified multirule procedure was also tested and was found to be ineffective. The authors recommend the multirule of Levy and colleagues but caution that its use may result in the detection of small shifts in the red blood cell count.