The paper is concerned with an analysis of errors and shortcomings that are typical of a method for the determination of 131I-triiodothyronine absorbed by erythrocytes using washing (the so-called Hamolsky test). To lessen or to prevent some of these errors as well as to reduce the time of investigation a new method is proposed where repeated centrifugation and washing of the blood samples will be of no use. This method was used simultaneously with the previous one in 71 patients of whom 31 formed a group of healthy persons. A study of each patient was doubled by taking a double blood sample: 4 ml instead of 2 ml. The dispersion (sigma 2) of the results obtained by the new method turned out to be less than in the dispersion of the standard method. Using the X2 criterion the results obtained by both methods were analysed. It has been established that at the level of the value p less than 0.001 between the first and second measurements there are no statistically significant differences that are so typical of the previous method. The dispersion and nonparametric statistical analyses after the use of the F and X2 criteria have shown a greater reproducibility of the results obtained by the new method.