The characteristics of spontaneous platelet aggregation (SPA) in a hereditary giant platelet syndrome (Montreal platelet syndrome, MPS) are examined. SPA was quantitated by microscopy from the decrease in single platelets in platelet-rich plasma (PRP). In contrast to normal donors, a significant proportion (20-50%) of platelets in MPS whole blood and PRP occurred in microaggregates typically containing 2-6 disk-shaped platelets. Stirring MPS-PRP at 1000 rpm for 10 minutes further increased the fraction of platelets in aggregates by 10-170%, the percentage increase not being correlated to the donor's platelet count (5000-220,000 microliters-1). Normal platelets resuspended in MPS platelet-poor plasma (PPP) did not undergo SPA, whereas MPS platelets resuspended in normal PPP or Ca2+-free, fibrinogen-free Tyrode's continued to show SPA. The increase in SPA could be inhibited by 10 microM prostaglandin (PG) E1, 150 mM ASA or glutaraldehyde or formaldehyde fixation; however, it was not inhibited by 10 nM PGI2 and was only partially inhibited by 1 microM 2-chloroadenosine and 1-10 units/ml apyrase. SPA in Acid-citrate-dextrose-PRP was much less than in PRP; however, SPA reoccurred on returning the platelets to platelet-free plasma or Tyrode's. Platelet aggregation (PA) could be increased over that due to SPA alone by the addition of adenosine diphosphate, adrenaline, collagen, ionophore A-23187, arachidonic acid and ristocetin, with results suggesting that the response to these agents is normal. The ristocetin-induced increase in PA was completely blocked by an IgG specific for Bernard-Soulier syndrome. In contrast, MPS platelets had a reduced sensitivity to thrombin, which appeared to be more pronounced at low platelet counts. There was no correlation between the thrombin insensitivity and the extent of SPA. Total adenosine triphosphate (ATP) and thrombin-induced release of ATP and platelet factor 4 appeared normal for MPS platelets. The ultrastructural features of MPS platelets were within normal limits except for an increased frequency of giant granules. SPA was observed for 5/5 MPS donors, but only one of three MPS donors' platelets evaluated for glycoprotein I and sialic acid content showed any measurable reduction as compared with normal controls. The above observations point to the existence of an as yet undetermined anomaly of MPS plasma membrane related to a fibrinogen and Ca2+ independent form of platelet aggregation.