Unknown specimens containing bilirubin in concentrations of 0.5 to 1.0, 5.2, 10, 15, and 20 mg/100 ml were tested in 13 laboratories. Analysis of the data suggests that bilirubin results reported from these laboratories will vary as much as ± 3.6 mg/100 ml from the true values. The means from the individual institutions differed widely from the true values, but the scatter of values about these individual means was small. When the data were pooled, the means were close to the true concentrations of the samples analyzed, but the variances were large. The hypothesis that incorrect bilirubin standardization was a significant cause of inaccurate results is presented.