Methods are described for the evaluation in vitro of the effect of tuberculin on the leucocytes of peripheral blood.

Washed leucocytes from a tuberculin sensitive host suspended in normal plasma are not lysed by tuberculin until after several hours of contact.

Washed leucocytes from a tuberculin-sensitive host slowly release into normal plasma a factor which will cause the lysis of normal leucocytes exposed to tuberculin.

Dialysis of normal plasma containing shed plasma factor causes the latter to precipitate with the euglobulins.

Shed plasma factor can be recovered from normal plasma which has been incubated with lymphocytes from tuberculin-sensitive hosts. Suspensions of neutrophils do not yield shed plasma factor.

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