A simplified and compact agitator for growth inhibition tests with serum-leucocyte mixtures has been described.

Several modifications have been made as well in the technique of the test, which have eliminated occasional irregularities that necessitated discarding the results of the individual experiment. Such irregularities were found to be due chiefly to injury of the pneumococci brought about by prolonged suspension in gelatin-Locke's solution which resulted in failure of the organisms to grow in the control serum-leucocyte tubes. This deterioration of the pneumococcus suspension may be greatly lessened or entirely prevented by the addition of a small quantity of a balanced phosphate mixture to the gelatin-Locke's solution. The use of small tubes made of Pyrex glass has also eliminated the former, not infrequent, occurrence of early hemolysis which was sometimes intense enough to disturb the results of the test.

It has been found that washed rabbit leucocytes, suspended in their homologous serum, may be kept in the ice box for as long a period as 2 days without showing any apparent diminution of their functional activity in the serum-leucocyte test.

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