The sensitivity of the immunoradiometric kit "RIA-gnost hTSH" (Behring) is poorer with a common automated pipetting station than with careful manual pipetting, principally because of tracer contamination of the inner wall of the tube, above the level reached by the wash solution. This can be eliminated by adapting the wash buffer volume so as to fill the entire tube. In this way, one can detect as little thyrotropin as 0.02 milli-int. unit/L, which is better than the 0.03 milli-int. unit/L claimed by the manufacturer. Cross-contamination from pipetting can induce overestimation of "suppressed" thyrotropin values. Prolonging the counting time to obtain an optimal counting error is of no practical value in the thyrotropin range where activity is low.