Sera from 93 patients with systemic scleroderma including incipient or prodromal acroscleroderma and from 43 healthy individuals were studied for cytotoxic effects on endothelial cells by means of three different in vitro methods. Inhibition of 3H-thymidine incorporation by endothelium was caused by about 33% of sera, almost exclusively from patients with diffuse scleroderma and the transitional form: acroscleroderma--diffuse scleroderma. An antibody-dependent cellular cytotoxicity assay revealed cytotoxicity of about 37% of sera from patients with diffuse scleroderma and the transitional form but also of a proportion of sera from patients with CREST syndrome with pronounced vascular changes. Serum cytotoxic activity, revealed by both methods, was related with more frequent involvement of muscle and kidney in the patients. A direct 51Cr release assay showed the cytotoxicity only in 4 of 68 cases in diffuse scleroderma with pronounced internal organ involvement. Thus, depending on the method used, various types of cytotoxicity could be detected in sera from patients with systemic scleroderma.