We examined the effects of a small initial sensitizing dose of antigen (dinitrochlorobenzene, DNCB) on the subsequent response to a second, defined sensitizing stimulus. The second stimulus was actually the regimen of four doses of DNCB (3.125, 6.25, 12.5, and 25 micrograms) normally used as the elicitation challenge. In two separate experiments 13 and 18 control subjects received an initial 'challenge' with the four doses to induce sensitivity, and 4 weeks later their responses were determined with a second, elicitation challenge. Two groups of 12 and 15 experimental subjects received an initial dose predicted to induce clinically detectable sensitivity in 50% or 25%, respectively. Four weeks later, their responsiveness was determined with quantitative challenge and the subjects who gave no response received a further challenge 4 weeks later. Their responses, compared with those from the control subjects, were augmented, indicating that sub-clinical priming of the immune system had indeed occurred.