The correct interpretation of a measurement result requires knowledge about its uncertainty. Depending on the conditions under which the analyst,is operating, different operational definitions of uncertainty have been proposed. They include: within-laboratory uncertainty, reproducibility uncertainty, bias-included uncertainty and absolute uncertainty. Here we consider the evaluation of the reproducibility uncertainty derived from the results obtained in an inter-laboratory experiment. Nine laboratories participated in an inter-laboratory study for the analysis of phenoxymethylpenicillin. The analyses consisted of a Karl-Fischer water determination, an acid-base titration to assay phenoxymethylpenicillin and a liquid chromatography (LC) method to determine 4-hydroxyphenoxymethylpenicillin and other impurities. The experimental set-up allowed to obtain for each determination s(r)(2) and s(L)(2) as estimates of the repeatability variance (sigma(r)(2)) and the between-laboratory variance (sigma(L)(2)), respectively. The reproducibility uncertainties for the different assays were then derived from these estimates. (C) 2003 Elsevier Science B.V. All rights reserved.