Quantitative techniques of sensitization to 2,4-dinitrochlorobenzene (DNCB) was used to determine in psoriasis the intensity and frequency of allergic reactions to DNCB following primary challenge with 2,000 μg allergen and secondary challenge with decreasing doses of DNCB. 56 patients with psoriasis and 23 healthy volunteers were examined. Frequency of positive reactions to DNCB was similar in both groups, since all normal controls were sensitized, whereas only 8 of 56 psoriasis cases failed to develop delayed hypersensitivity to DNCB. However, the intensity of acquired contact allergy was significantly diminished in psoriasis in comparison with controls. The patients with stationary skin lesions resembled the normal population in the intensity of reaction to DNCB. Decreased intensity of DNCB sensitization seemed to be related to the activity of the disease, but not correlated with the extent of the lesions. A relationship was found between reduced reactivity to DNCB and decrease in E rosette-forming lymphocytes. The data suggest that the impaired function of T lymphocytes in active psoriasis could be responsible for both, defective recognition of contact antigens, such as DNCB, and the alteration of secondary response to DNCB