A new measuring device for the estimation of the "spontaneous" aggregating activity of thrombocytes has been developed. In this photometric platelet aggregation test (PAT III) a small amount (0.6 ml) of platelet-rich plasma (PRP) is being rotated in a disc-shaped cuvette at 20 rpm, at 37% C. Changes in optical density of PRP which are induced by the formation of platelet aggregates are continuously registered using a chart recorder. The decisive trigger mechanism for aggregation is an increase of plasma pH in the rotating sample which is caused by evaporation of CO2. The results of the test depend on the platelet count in PRP. Aggregation curves are misrepresented by admixture of erythrocytes and lipid turbidity. The tendency of platelets to aggregate increases within 60-90 min following blood sampling. During this period the time interval to the onset of aggregation(Tr) is shortening, and the maximum aggregation speed (alpha2) is increasing. The spontaneously enhanced aggregation tendency of thrombocytes may be reliably measured from 60 min after drawing the blood. The reason for these time-dependent changes which are also demonstrable in ADP-collagen-or epinephrine-induced aggregation is probably the primary shape change of platelets, which occurs after blood drawing and makes them "stickly" and aggregable. PAT III was developed for the detection of enhanced platelet aggregation, indicating a risk of thrombosis and thromboembolic omplications. The new measuring device has been designed as "universal" aggregometer. Additional equipment is available for the registration of ADP-collagen-or epinephrine-induced aggregation similar to Born's and O'Brien's methods. The device may be mounted easily on an Eppendorf photometer without further modifications.