Tuberculin from the human and from the bovine type of tubercle bacilli inhibits the growth of cells from explants of bone marrow, spleen and testes of tuberculous guinea pigs, and is toxic for these cells, but has no effect on explants of the same tissues from non-tuberculous animals.

"Tuberculin" from other acid-fast bacteria has no inhibitory or toxic action on explants of tissues from either tuberculous or non-tuberculous guinea pigs.

Tuberculins from the avian, bovine and human types of tubercle bacillus as well as "tuberculin" prepared from the Duval and from the Kedrowsky strains of M. leprae inhibit the growth of the cells of explants of the spleen and bone marrow of tuberculous fowls and are toxic for these cells, but have no effect on the explants from tissues of non-tuberculous chickens.

"Tuberculins" from other acid-fast bacteria have no effect on the growth of explants of tissues from tuberculous or from non-tuberculous fowls.

Tissue culture methods indicate that the sensitivity of tuberculous tissues to tuberculin is inherent in the cell, and that it cannot be passively transferred.

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