Some persons seem to lose long-standing peripheral hypersensitivity to tuberculin earlier than others. This was seen in a group of five healthy student nurses who had been BCG-vaccinated as children and were found, in routine skin testing, to be negative to 100 TU of tuberculin purified protein derivative (PPD). They were revaccinated, which resulted in conversion to 10 TU skin test positivity. In agreement with this, their buffy coat cells achieved reactivity to 100 microgram/ml of PPD in a leucocyte migration inhibitory factor (LIF) assay. However, the LIF response, being maximal at 4 weeks, faded away earlier than skin reactivity. Peripheral blood lymphocyte proliferation was studied with several PPD concentrations, 10 microgram/ml always inducing the maximum 3H-thymidine uptake. This was still high at 6 months after vaccination, when the skin reactions tended to be smaller than earlier. The reason why the various parameters of cellular hypersensitivity followed different courses is not known, but it may involve different subpopulations of lymphocytes, activity of suppressor cells or influence by serum factors such as mycobacterial antigen-antibody complexes.