Nickel sulphate is a sensitizer in guinea pigs, but the frequency of sensitization varies from study to study. The dose‐response relationship for NiSO46H2O was evaluated in the guinea pig maximization test in this study, 6 intradermal (0.01%–3.0% aq.) and 6 topical (0.25% 10.0% pet.) concentrations were chosen for induction and NiSO46H2O 1% pet was used for challenge, based on the absence of skin irritation in a pilot study. Blind reading was performed. A logistic dose‐response model was applied to the challenge results. At 48 h, a linear relationship was obtained between the intradermal induction dose (but not topical dose) and the response, resulting in a maximum sensitization rate of 40% after 3% i.d. The reactivity disappeared at re‐challenge I week later. Following a booster closed patch on day 35, using NiSO4pet., the animals were challenged with NiSO42% pet. and statistical analyses of 72‐h readings revealed a non‐linear dose‐response relationship, giving a maximum response Frequency of 40% after initial induction with NiSO4, 3% i.d. a