Blood lymphocytes from 37 patients with systemic sclerosis were characterised using monoclonal antibodies in a two colour flow cytometric (fluorescence activated cell sorter (FACS)) analysis. The ratio of helper CD4+ to suppressor/cytotoxic CD8+ T cells was raised in patients compared with that in 30 healthy controls owing to decreased CD8+ cells. In the patients CD4+ and CD8+ cells displayed an increased expression of the activation marker HLA-DR. The relative number of CD11b+ CD8+ lymphocytes (suppressor T cells) was normal, but the calculated absolute counts of this cell type were slightly reduced. The proportions and absolute numbers of suppressor inducer T cells, defined as CD45R+ CD4+ cells, were on average only half the levels observed in controls. These findings were not related to the inflammatory activity as measured by acute phase plasma proteins or serum immunoglobulins. Activated T cells were seen at all stages of the sclerotic process and especially during the early stages of the disease and in patients who had suffered occupational exposure to silica dust. A high proportion of activated T cells was also linked with impaired small intestine function but not with the degree of skin or lung involvement. A loss of suppressor inducer T cells was more pronounced later in the disease and in patients with the CREST (calcinosis, Raynaud's phenomenon, oesophageal dysmotility, sclerodactyly, telangiectasia) syndrome. These data provide further evidence for an involvement of T cell mediated immunity in the perpetuation of systemic sclerosis.