Human late-phase allergic skin reactions (LPSR) are characterised histologically by a mixed infiltrate of granulocytes and mononuclear cells. Quantitative immunohistology reveals a correlation between the numbers of CD4+ T-cells and the number of activated eosinophils at LPSR sites. These CD4+ T-cells are predominantly CD45RO+ and belong to the ‘memory’ T-cell subset. Immunofluorescence studies indicate that type III (immune complex) mechanisms make no significant contribution to the LPSR. On the other hand, the LPSR and classical delayed-type hypersensitivity (DHT) both have a marked CD4+ T-cell component. T-cell infiltration is a more diffuse process in DTH than in the LPSR; between 24 and 48 h, DTH sites show a further increase in T-cell numbers with the appearance of CD8+ T-cells and an influx of monocytes. Activated eosinophils are present in the early stages of both LPSR and DTH, but are more prominent in the LPSR. These different patterns of cellular infiltration and activation are probably due to differences in the cytokine secretion pattern of the infiltrating T-cells. Detailed analysis of this hypothesis will require the use of in situ hybridization techniques.