We have investigated the contributions of dose per unit area and area of application to the degree of sensitivity induced by dinitrochlorobenzene (DNCB). Subjects sensitized with 35.4 micrograms/cm2 showed equal sensitization whether the total dose was 250 micrograms applied to 7.1 cm2 or 62.5 micrograms applied to 1.8 cm2. Three groups of subjects were sensitized with 16.4 micrograms/cm2 applied as 56 micrograms on 3.5 cm2, 116 micrograms on 7.1 cm2 or 232 micrograms on 14.2 cm2. Although the total dose changed by a factor of four, the proportions of subjects sensitized were comparable (66, 50 and 55% respectively), and their degrees of reactivity were similar. This contrasts with previous findings that when the area of sensitization was fixed and the concentration was increased over a similar range, highly significant increases in the proportion of sensitized subjects and degree of reactivity occurred.