Univalent and bivalent (2Fab'-Fc) chimeric rat-human antibodies were constructed by chemical coupling (thioether bonds) of Fab' fragments from the 33B31 rat anti-human interleukin 2 receptor (alpha chain) monoclonal antibody to human IgG or Fc fragments. The purity of the chimeric antibodies obtained after purification was assessed by the sodium dodecyl sulfide-polyacrylamide gel electrophoresis method. The affinity of chimeric antibodies for the human interleukin 2 receptor was determined. The affinity of the 2Fab'-Fc was better (Kd = 0.87 nM) than that of the Fab'-IgG (Kd = 2.04 nM) or the Fab'-Fc chimera (Kd = 3.93 nM). Binding studies on a T-cell clone showed that the percentage of positive cells recognized by chimeric antibodies was comparable to that obtained with unmodified 33B3.1 IgG2a or its corresponding Fab' fragment. In addition, the inhibition of interleukin 2-induced cell proliferation and allogeneic proliferative response by chimeric antibodies was of the same magnitude as that obtained with the rat IgG2a anti-interleukin 2 receptor monoclonal antibody and Fab' fragments. This study shows the possibility of changing the isotype of monoclonal antibodies without important modification to their binding activity. These reagents may offer an alternative to unmodified monoclonal antibodies for therapeutic application.