SUMMARYThree tissue-typing workshops were organized to coordinate and evaluate tissue typing in the Southeastern Regional Organ Procurement Program. Although all participating laboratories employed a cytotoxic assay, considerable variability among protocols was found at the first workshop, resulting in poor correlation of results. In the later workshops a standard protocol was followed by all participants, and improvements in reproducibility and degree of interlab correlations were observed. It was concluded that even minor variability in technique, whether deliberate or inadvertent, may result in variable typing results.