Twelve subjects sensitive to nickel underwent patch testing with serial dilutions of nickel sulfate in petrolatum and in water. The provocation threshold (the lowest amount of nickel producing a reaction) varied from 5.2 mg (2.5%) to 0.47 microgram (0.01%) when tested in petrolatum. The provocation threshold was higher with aqueous solutions. The lowest provocation threshold in a statistically significant number of individuals was 1.5 micrograms. Nickel bioavailability from dermatitis-inducing earrings and a necklace was determined by immersing them in plasma, normal saline solution, and synthetic sweat at different pH values over 7 days. Nickel leaching under these conditions exceeded the provocation threshold 1.4- to 93-fold, depending on object and solution. The time course varied with solution. After leaching, surface nickel was measured by the action of dilute nitric acid and total nickel was determined by complete dissolution in aqua regia. Nickel was more bioavailable from earrings than from the necklace, presumably reflecting different metallurgy. Plasma was the most effective solution for removing available nickel from earrings, a possible explanation for the frequent induction of sensitization by ear piercing. Comparison of provocation threshold and leaching results may help determine risks from consumer products.