The effect of caeruloplasmin levels on the sensitivity for activated protein C (APC), measured by a clotting assay based on the activated partial thromboplastin time, was investigated in a large group of healthy individuals without factor V Leiden. A modest inverse association between caeruloplasmin and normalized APC sensitivity ratio was found (regression coefficient beta = -0.33 x 10-2; 95% confidence interval, -0.42 x 10-2 to -0.24 x 10-2). After adjustment for sex and oral contraceptive use, this association weakened (beta = -0.19 x 10-2; 95% CI: -0.34 x 10-2 to -0.05 x 10-2). After additional adjustment for factor VIII levels, which are known to influence the assay, the effect of caeruloplasmin on APC sensitivity completely disappeared.