To determine the concentration of antibody solution and other protein solutions, the absorbance at 280 nm is generally measured. The aim of this study is to confirm whether the absorption is measured correctly, to understand the actual situation and to improve the measurement accuracy. In order to investigate these issues, monoclonal antibody solution was distributed to 34 participants and conducted an interlaboratory comparison on the absorbance measurement. The measurement results reported by participants were widely distributed showing 9.65 % as the relative standard deviation (CV). In the second-trial case with 32 participants, the CV was resulted in 11.6 % which was larger than the previous results. With the use of z-score analysis and a survey of participants, the significance of the establishment of measurement procedures was suggested. The third trial was performed by selecting 7 participants which were reported valid and outlier results in the previous comparisons. The comparison result reported by their own measurements was 2.99 % in the CV. Subsequently, the measurements were performed according to strictly following the measurement procedure including calibration by the same 7 participants. The measurement results showed that the CV was decreased to 0.906 %, and in good agreement. Based on the results over three interlaboratory comparisons, factors involved in achieving accurate measurements were presented.