['Contact sensitization by ingredients in personal products is an important clinical problem. It is not clear how sensitization is induced by the generally low concentrations at which they occur but it might be the result of repeated exposure.', 'To compare the strength of contact sensitization induced by a single exposure to 2,4-dinitrochlorobenzene (DNCB) (60 microg cm(-2)) or three repeated exposures to a subsensitizing dose (10 microg cm(-2)).', 'Two groups (n = 10) of healthy adult volunteers were randomized to receive either a single patch of DNCB 60 microg cm(-2) or three once-weekly applications to the same site of 10 microg cm(-2) DCNB. Four weeks after the last application, sensitization was quantified by measurement of responses (skinfold thickness) to a graded series of four challenge doses.', 'All the volunteers were sensitized and the strength of the responses was virtually identical between the groups.'] Is repeated low-dose skin exposure an effective sensitizing stimulus , a factor to be taken into account in predicting sensitization risk?