Experiments in passive transfer of nickel allergy require highly sensitive guinea pigs and a reliable and technically usable test method. A comparison was made between two sensitization methods, the guinea pig maximization test of Magnusson and Kligman (MK) and a combined injection and painting method devised by Polak and Turk (PT). Intradermal injection was chosen as test method. The reactions to the test were greater than those of normal animals and the differences were statistically significant. With testing before the start of sensitization it was found difficult to obtain highly sensitive animals (tolerance). PT painting resulted in rather higher sensitivity with Triton X-100 than with sodium lauryl sulphate. In a comparison between the MK and PT methods, no difference in effectiveness was found provided that the animals had not been pretested. Repeated PT injections and paintings caused a further increase of the sensitivity, but the toxicity of nickel is a limiting factor.