The effects of experimental infection of calves with Sarcocystis cruzi on the blood coagulation cascade were investigated. Calves were inoculated orally with 1 x 10(5) sporocysts (group S1, n = 6) or with 5 x 10(5) sporocysts of S. cruzi (group S2, n = 3). A group of eight calves served as non-infected controls (group C). The animals were bled once during the first 4 weeks of infection and twice a week thereafter until day 40 p.i. The following parameters were measured: activated partial thromboplastin time, prothrombin time, thrombin time, reptilase time, thrombin coagulase time, factors XII, XI, X, IX, VIII:C, VII, V, prekallikrein, fibrinogen, alpha 2-antiplasmin, antithrombin III, alpha 1-antitrypsin, alpha 2-macroglobulin, haematocrit, haemoglobin, numbers of erythrocytes and thrombocytes. The infected calves developed acute sarcocystiosis from 25 (S1) or 29 (S2) days p.i. onwards. During the acute disease, antiproteases tended to elevated values and thrombocyte counts were generally reduced. In group S1 prolonged prothrombin time and reduced activities of factors VII or V were found. In group S2 accelerated prothrombin time and activated partial thromboplastin time, as well as elevated factor X activities, were recorded even before the onset of clinical disease at 19 days p.i. While prothrombin time returned to normal levels thereafter, activated partial thromboplastin time remained short. Activities of factor V, factor VII and factor X were significantly reduced in group S2 at the onset of acute sarcocystiosis, and one of the three calves died at 29 days p.i. The other parameters were not significantly affected by either dose of infection. No evidence for a classical disseminated intravascular coagulation syndrome could be found; however, it was demonstrated that S. cruzi alters plasma coagulation in a dose-dependent way.