Human volunteers were sensitized to dinitrochlorobenzene (DNCB) applied to the skin in a single 1.5-mg dose in acetone, and extracts from their leukocytes were injected into human recipients. Positive delayed hypersensitivity reactions to DNCB were observed both in recipients of extracts and in controls, but there was a slight difference between the two groups. Moreover, the capacity of extracts to transfer – or enhance – sensitivity to DNCB appeared to be correlated to the degree of sensitivity of donors.