A sealed vacuum extraction method (Seditainer) for determining the erythrocyte sedimentation rate (ESR) was compared with the standard Westergren ESR technique. The Seditainer method was particularly easy to use, showed acceptable precision, reduced the biohazard risk to laboratory staff and, on storage of sealed blood specimens for 24 hours at 4 degrees C, gave ESR values that had decreased by a mean of only 1.9% (95% CI + 0.2 to -4.0%). Seditainer tubes are shorter (100 mm) than Westergren tubes (200 mm) and this reduced test sensitivity at ESR values above 55 mm/first hour. After application of the manufacturer's correction formula to adjust for tube length there was improved correlation (n = 150, r = 0.936, p less than 0.001), but still considerable scatter, between the Seditainer and Westergren results. Seditainer ESR values above 55 mm/first hour should therefore be regarded as semiquantitative. This vacuum extraction method offers a simple and safe technique for measuring the ESR and specimens can be stored overnight at 4 degrees C.