Evidence is presented, confirming the presence of an anticoagulant sys
tem in the plasma of Atlantic salmon (Salmo salar L) and rainbow trout
(Oncorhynchus mykiss Walbaum) (Order: TL Teleostei. Family: Salmonida
e) that bears striking similarities with the protein C anticoagulant s
ystem in mammals; its vitamin K-dependence was documented through a wa
rfarin feeding trial. A potent activator of this system is the protein
C activator from the venom of the Central American Moccasin, Agkistro
don bilineatus. When activated, the system splits the tripeptide subst
rate glu-pro-arg-pNa, which is the substrate preferred for the in vitr
o assay of human protein C. It also prolongates the plasma activated p
artial thromboplastin time, indicating that the system is of clinical
relevance. A temperature dependence of the plasma protein C-like activ
ity was evident, the mean activity being 5- to 10-fold higher, but als
o more variable. in both species in summer and early fall, than it was
in winter. There was also a species difference, with rainbow trout ha
ving the higher levels. In man, subnormal values of protein C implies
an increased risk of thrombosis. Whether this applies to fsh is not kn
own. It is, however, a fact that microvascular thromboses are prevalen
t in farmed Atlantic salmon in winter, whereas thrombotic disease is n
ot a problem in rainbow trout; in the present study plasma protein C-l
ike activity was 30% (of a human reference plasma) in salmon at 4 degr
ees C compared to a level of 60% in rainbow trout. A complicating fact
or for the assay of protein C-like activity in salmonid plasma, is the
poor stability of the inhibitory system upon storage. Consequently, a
ssays have to be done with freshly prepared citrated plasma.