Résumé : The reactivity and specificity of potyctonal and monoclonal anti‐idiotypic antibodies raised against monoclonal anti‐progesterone and anti‐arsonate antibodies have been studied by solid phase radioimmunoassay (RIA) with immobilized idiotype and by passive haemagglutination with idiotype‐coupled red cells. The sensitivity of the two methods was comparable, though some cross‐reactions were only detected by RIA. Passive haemagglutination was found to be especially suitable in screening for monoclonal anti‐idiotypes in hybridoma supernatants and ascites. and had advantages over RIA in detection of syngeneic anti‐idiotypes. Demonstration of binding site‐associated idiotopes was possible by haemagglutination inhibition. RIA and haemagglutination were used to investigate the idiotypic relationships between BALB/c antiprogesterone and anti‐arsonate monoclonal antibodies which share heavy chains encoded by VHIX variable region genes. Copyright © 1987, Wiley Blackwell. All rights reserved