An examination of 96 patients has established that the development of a pyo-destructive process in the lungs and pleura is accompanied by an increase of concentration of circulating thrombocytes as well as their adhesive-aggregating activity and degenerative-dystrophic alterations. These parameters were most pronounced in patients 1-3 days before their sudden death from thromboembolism of the pulmonary artery without a clinically detected source. Persantine, aspirin, pyrazolones and heparin in generally accepted therapeutic doses were shown not to possess evident anti-aggregative properties.