Two groups, each of six guinea pigs, were sensitised by the application of cobalt chloride (CoCl2) on the skin on day 0, 2, 7, and 9 and the establishment of contact allergy was confirmed by patch testing on day 21. A further six animals were not sensitised. Starting on day 42 one sensitised group and the non-sensitised group were exposed by inhalation to 2.4 (0.8) mg (mean (SD)) Co in the form of CoCl2 for six hours a day for two weeks. After exposure the lungs were lavaged and the cells obtained were studied by light and electron microscopy. In the sensitised exposed group much more lavage liquid was retained in the lungs than in the other two groups; although more liquid was instilled in the lungs of this group, on average only 5 (range 2.5-10) ml were recovered compared with 10 ml in all animals in the other two groups. In the sensitised exposed group, the percentage of neutrophils and eosinophils tended to be higher than in the non-sensitised exposed group. The results indicate that the lungs of guinea pigs allergic to contact with Co react differently to inhaled Co compared with those of non-sensitised ones.