To analyze the structure of LH in three patients with immunologically anomalous LH, the whole coding region of the LH beta-subunit gene was examined. These patients were infertile, and their serum LH levels could not be measured with an immunoassay kit. Immunoblotting of the LH beta-subunit showed no marked changes in the molecular size of LH beta. Genomic DNA was extracted from peripheral lymphocytes of the patients and normal controls, and LH beta genes were amplified by the polymerase chain reaction technique, using primer pairs that are capable of specifically amplifying only the LH beta gene without interference by the CG beta genes. No deletions were observed in the coding regions of the LH beta gene of the patients. Nucleotide sequencing revealed two nucleotide substitutions in the LH beta gene of the patients, which cause amino acid replacements from Trp8 (TGG) to Arg8 (CGG) and Ile15 (ATC) to Thr15 (ACC). Restriction fragment length polymorphism analysis in three families indicated that the affected probands were homozygous, and their family members were heterozygous, except for their husbands. The heterozygotes showed reduced detectability with the LH immunoassay kit. These results suggest that these amino acid replacements are responsible for this immunologically anomalous variant.