No appraisable and exact procedure for assessing the titer of hemolytic antibody (hemolysin) has been devised yet in teleosts including salmonid fishes. A device for hemolysin titratin was undertaken here by applying ralnbow trout hemolysin and complement. Hemolytic complement activity (CH
50 unit) of pooled normal serum was determined as 32.5 CH
50 units/m
l by reacting goldfish red blood cell (GFRBC) sensitized with antI-GFRBC hemolysin (inactivated by heating at 44°C for 20 min) and definitely diluted complement at 30°C for 60 min. From the decided CH
50 unit value, the dilution of the complement for the preparation of complement bearing 2 CH
50 units was calculated at 1:44, and this diluted complement possessed an ability to hemolyze 100% of 1×10
7 sensitized GFRBC when this complement of 0.5 m
l was added to the total reaction volume of 1.5 m
l. In addition, this complement revealed hardly any spontaneous or non-specific hemolysis activity against unsensitized GFRBC under same conditions. Hemolysis was assayed by incubating a hemolytic system consisting of GFRBC, serially diluted hemolysin and complement bearing 2 CH
50 units at 30°C for 60 min. Subsequently, hemolysin titer expressing 50% hemolysis against GFRBC was estimated from hemoilysis rates. In these assaying arrangements, hemolysin titration could be accmplished without trouble in rainbow trout immunized with the intramuscular (
i.
m.) and intraperitoneal (
i.
p.) injections of 10
8 GFRBC per a juvenile. Fish in the
i.
p. injection produced a higher level of hemolysin titers than one in the
i.
m. injection in the titration aftet 1 week. Hemolysin samples, which were taken out from primed (
i.
p.) fish group held at a water temperature of 10°C, drastically enhanced the values to a high of 641±251 (mean± standard deviatin, n=5) after 5 days. In succession, hemolysin titers rapidly descended to 485±213 after 7 days through 55±21 after 15 days hemolytic activity became undetectable after 20 days. From the results of hemolysin titration, it was coneluded that the primary antibody response of primed rainbow trout is markedly transitory.
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