Most cases of primary nickel sensitization are caused by occluded exposure to nickel released from earrings, buttons, etc. Nickel-sensitive females are both domestically and occupationally exposed to trace amounts of nickel. The present study was designed to define the minimum eliciting threshold concentration of nickel exposure in previously nickel-sensitized individuals, after open application on normal forearm skin with and without surfactant. A dilution series of NiCl2, ranging from 0.1 to 4000 ppm nickel with and without 4% sodium lauryl sulfate, was applied to healthy volar forearm skin. None reacted with definite allergic reactions to 100 ppm and below. Follicular reactions were seen in the concentration range from 0.1-100 ppm in a few patients and 1 healthy control. The results are discussed in relation to consumer products containing 1-10 ppm nickel.