AbstractThe cross‐reaction between HCG and HLH was studied by radioimmunological techniques using 43 anti‐HCG sera. With some of them the cross‐reaction between HCG and HLH was complete as shown by the similar inhibition of binding of labelled HCG and HLH to antibody obtained with both hormones. A few antisera failed to react identically with labelled HCG in the presence of unlabelled HLH and HCG, but with labelled HLH identical inhibition slopes were yielded by unlabelled HLH and HCG. Moreover, with one antisera labeled HLH bound poorly to antibody and unlabelled HLH produced only a slight decrease in the percentage of labelled HCG bound to antibody. These experiments indicate that the cross‐reaction between HCG and HLH is incomplete and that there are immonologically active sites which are specific for HCG and which are not possessed