Fibrinolytic activity (FA) is one of a number of measures of haemostatic function being carried out in a prospective study of clinical thrombotic disease, including ischaemic heart disease (IHD), in an industrial group in North-West London (Meade, 1973). FA is measured by the dilute blood clot lysis time of Fearnley et al. (1962), and is expressed as 1/lysis time in hours x 100. Recruitment into the study is in a random order from a nominal roll; the response rate is about 80%. All participants working on days are seen between 0900 and 1030 hours. Venepuncture is performed without compression of the arm after a 10–15 minute period of rest; participants in whom venipuncture was difficult have been excluded from the analyses which follow. FA is significantly greater in black men than white men (but there is no difference between black and white women); results from black participants have therefore also been excluded. Findings deal with 625 day-men aged 18–64, and 290 day-women aged 18–59; of the women, 43 were on oral contraceptives when recruited.