Following preliminary in-vitro experiments, platelet-leukocyte conjugates and their determinants were evaluated in citrated whole blood from 349 subjects (209 women, age 16-92 years) randomly recruited from the general population. Platelet activation by ADP/collagen but not leukocyte stimulation by fMLP or LTB4 resulted in formation of platelet conjugates with PMN or monocytes. In the population study, mixed cell conjugates, platelet P-selectin and leukocyte CD11b were measured by flow cytometry both at baseline and after in-vitro stimulation with ADP/collagen. The latter significantly increased platelet conjugates with either PMN or monocytes, platelet P-selectin and leukocyte CD11b expression. Platelet count significantly correlated with platelet-PMN, platelet-monocyte conjugates and P-selectin both at baseline and upon stimulation. In all conditions, both conjugate levels correlated with each other, when adjusted for gender, age and platelet count. Age correlated with platelet-PMN conjugate numbers in basal and stimulated conditions and with basal P-selectin. ADP/collagen stimulation resulted in higher P-selectin and conjugates values in women. Among risk factors, a significant correlation was found between conjugate and glucose levels. In conclusion, the presence and formation in whole blood from a large population of platelet-leukocyte conjugates reflects primary platelet - but not leukocyte - activation and varies with gender, age, platelet count and blood glucose.