Normal plasma contains nonsedlmentable platelet factor-3 (NS-PF3) activity, thought to be caused by circulating platelet membrane fragments. Stypven time (ST), an assay for PF-3 activity, of plasma prepared by differential centrifugation and by filtration through 0.22 μ Millipore filters were Investigated. The average ST for platelet-rich plasma (PRP), low-spin platelet-poor plasma (LSPPP), medium-spin PPP (MSPPP), high-spin PPP (HSPPP) and filtered PPP (FPPP) was 28.0, 40.4, 43.4, 61.7 and 65.5 sec, respectively (27 determinations). Filtration of plasma did not affect factor V and X activities. Material eluted from filters after filtration of plasma consisted of membrane vesicles with high PF-3 activity. ST were then measured in plasma preparations obtained from smoking (S) (>15 cigarettes/day) and nonsmoking (NS) healthy male individuals, ages (A) between 45-64. Data obtained were grouped according to age and smoking habits (Gr. I, 9 S, A 45-64; Gr. II, 14 NS, A 45-54; Gr. III, 7 S, A 55-64; Gr. IV, 14 S, A 55-64) and subjected to two-way analysis variance employing the BMDP2V program. Significant shortening of ST was noted in LSPPP (p<0.02) and MSPPP (p<0.02) in smoking groups which, however, showed no significant differences in PRP (p>0.8), HSPPP (p>0.06) and FPPP (p>0.4). Results suggest smoking individuals exhibit significantly higher NS-PF3 activity in plasma.