Indices of acute in vivo platelet activation in response to cigarette smoking were sought in 10 healthy subjects. Judging by platelet aggregate ratio, platelet factor 4 and beta-thromboglobulin measurements, platelet activation took place from 10 to 30 min after smoking in most subjects. These signs correlated well with the results of scintigraphic studies showing signs of accumulation of 111-In-labelled platelets in the spleen in response to smoking.