The one-stage prothrombin time test was analyzed to study the influence on the total variance of within-run variation and among-run (day) variation over 10 nonconsecutive days, difference between fibrometers, and variation among four commercially available thromboplastins. The variation among thromboplastins was found to be a major component of the total variance. Among-run (day) variance, difference between fibrometers, and a thromboplastin-run (day) interaction were each found to be statistically significant, but their influence was relatively small compared with that of thromboplastin variance. These influences were more apparent when a plasma with a prolonged clotting time was used.