The possibility of reduction of numbers of fresh coumarin and normal plasmas has been studied in a multicentre manual prothrombin (PT) calibration of high international sensitivity index (ISI) rabbit and low ISI human reference thromboplastins at 14 laboratories. The number of calibrant plasmas was reduced progressively by a computer program which generated random numbers to provide 1000 different selections for each reduced sample at each participant laboratory. Results were compared with those of the full set of 20 normal and 60 coumarin plasma calibrations. With the human reagent, 20 coumarins and seven normals still achieved the W.H.O. precision limit (3% CV of the slope), but with the rabbit reagent reduction < 50 coumarins with 17 normal plasmas led to unacceptable CV. Little reduction of numbers from the full set of 80 fresh plasmas appears advisable. For maximum confidence, when calibrating the ISI of a new reagent, it would always seem worthwhile to use the full number of patients' and normal plasmas at each centre for the initial calibration, i.e. 60 fresh coumarin and 20 fresh normal plasmas. Smaller numbers should achieve the required precision with the lower ISI reagent.