Immunoassays are widely used for measuring gonadotropins, including luteinizing hormone (LH), as these are both specific and sensitive. Because LH is microheterogeneous it has been claimed that the specificity of monoclonal antibodies used in two-site immunoassays can limit their clinical utility. Furthermore, we reported earlier a common genetic variant form of LH due to amino acid alterations in the LHbeta gene that is poorly or not recognized by antibodies directed against epitopes present in the intact molecule. We here report the result of an LH epitope mapping using 30 different monoclonal antibodies. The antigenic area affected by the amino acid alterations is fairly large, as antibodies to the two intact domains are able to sandwich each other. Combinations of alpha-beta or beta-beta antibodies generally provide alternatives for unbiased detection of circulating LH and some have reasonably good discrimination of human chorionic gonadotropin. The beta-beta combinations exhibit a peculiarity in urine determinations as they detect the urinary beta-core fragment. Our aims were to study the altered immunoreactivity caused by the amino acid changes and to design two-site LH assays fully capable of recognizing the biologically active LH variant. We also conclude that the variability in recognizing the universally occurring LH variant is the most important factor contributing to the widely documented LH immunoassay discrepancies.