In a collaborative study by eleven laboratories performed within the framework of the European Community Bureau of Reference (BCR), a third reference material for thromboplastin, rabbit, plain, has been calibrated against RBT/90, the current WHO international reference preparation for thromboplastin, rabbit, plain. This third reference material (coded CRM 149S) has a mean International Sensitivity Index (ISI) of 1.257 with a standard error of the mean of 0.013. The previous BCR reference material for thromboplastin, rabbit, plain (coded CRM 149R) and the reference material for thromboplastin, bovine, combined (coded OBT/79) were also included in the trial for assessment of the long-term stability of the ISI values. The relationship between ISI values determined in the present study was nearly identical to that of the historical values. These results offer reassurance with regard to the long-term stability of these reference materials.