Fibroblastic cells derived from rheumatoid (RA) and nonrheumatoid (N-RA) synovial tissue (synovial cells) were used as targets in assays of antibody-dependent cell-mediated cytotoxicity (ADCC). Synovial cells that had been pretreated with human alloantisera were rapidly lysed by normal human blood mononuclear cells. RA and N-RA synovial cells were equally susceptible to ADCC under these assay conditions. Antibody to autologous synovial cells was not detected in sera from 10 patients with RA by this method of assay.