1. Platelet activation status and response to stimulation with agonists ex vivo were studied by whole blood flow cytometry in 15 women with pre-eclampsia, 20 age- and gestational-age-matched women who completed a normal pregnancy, and 20 age-matched non-pregnant women. 2. Women with proteinuric pre-eclampsia showed evidence of activated, degranulated platelets in the circulation, with increased platelet-bound fibrinogen, increased expression of the lysosomal granule membrane antigen, CD63, and raised plasma levels of beta-thromboglobulin. 3. CD63 expression and beta-thromboglobulin per platelet were also significantly higher in normal pregnant women than in non-pregnant women, but in these subjects fibrinogen binding was normal. 4. There was good correlation for all subjects in degranulation, measured by CD63 antigen expression, and by plasma beta-thromboglobulin levels corrected for platelet count (r = 0.65; P < 0.01). 5. Platelet responsiveness to ADP in vitro showed a heightened degranulation response (CD63 expression) in normal pregnancy compared with the non-pregnant control group, which was increased further in women with non-proteinuric and proteinuric pre-eclampsia. 6. However, this response was not accompanied by an increased binding of fibrinogen to GPIIb-IIIa. Fibrinogen binding in response to 'weak' agonist stimulation, by low concentrations of ADP or, in a subgroup by adrenaline, was in fact lower in the normal pregnant women than in the non-pregnant women. 7. It is postulated that women at risk of developing pre-eclampsia may have hyper-reactive platelets, primed to undergo release by passage through the abnormal placenta.