Summary
Platelets washed by the albumen density gradient technique were used as the sole source
of thromboplastin in a new test, and also as the sole source of factor X in a modification
of the one-stage factor X assay test of Denson. These platelets were shown to be capable
of generating considerable amounts of thrombin during periods of preincubation of
up to 8 minutes. For at least 3 minutes of preincubation their size and shape remained
invariant, as measured by counting and by sizing on a Coulter channelyser. It is suggested
that this test measures a function of the platelet surface, rather than of the platelet
after viscous metamorphosis. This function is tentatively named platelet catalytic
activity (PCA). Support for this concept was given by the fact that aspirin and phenylbutazone
did not affect the test. The factor X assay system confirmed the presence of a small
amount of mostly inactive factor X on the surface of the platelet. This factor X could
be activated by Russell viper venom.
