On the theory that the enhancing effect of Ca++ on hemolytic activity in complement-antiserum-erythrocyte systems should also be present in immune treponemal immobilization, a series of experiments using known complement concentrations were performed. When complement concentrations and other factors were the same, immobilization titers were consistently increased by the addition of ionic calcium. This effect was accompanied by the presence of increased hemolytic activity in the residual test

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... tures after incubation. The addition of calcium had no adverse effects on the TPI test results or on treponemal survival.