SummaryBackgroundMethylisothiazolinone (MI), a preservative that is commonly used in personal care products, is now recognized as an important contact allergen in both cosmetic and occupational settings.ObjectivesTo analyse T lymphocyte responses to MI, in order to provide important information regarding the relationship between the nature of such responses and skin sensitization potency.MethodsProliferative responses to free MI and to an MI–human serum albumin (HSA) conjugate were measured according to [3H]thymidine incorporation (n = 56 donors; patch test scores of + in 20, ++ in 29, and +++ in 7). Peripheral blood mononuclear cells were cultured in the presence of MI (0.001–1 µg/ml) or MI–HSA (0.001–100 µg/ml). Proliferating CD4+ and CD8+ T lymphocytes were identified by flow cytometry with the intracellular marker Ki‐67.ResultsFor free MI, modest positive responses were recorded for 7 of 31 donors. In contrast, MI–HSA stimulated more marked responses in 17 of 31 donors. Characterization of positive proliferative responses showed variable patterns of proliferating CD4+ and CD8+ T lymphocytes from donors with the same patch test scores and similar maximal values.ConclusionsMI–HSA is able to induce secondary responses in lymphocytes drawn from sensitized subjects, and provides a more effective source of antigen than free MI. Furthermore, individual donors show differential activity profiles with respect to T lymphocyte subsets.