OBJECTIVE: To assess the LH response of the pituitary gland to GnRH stimulation in healthy women with a mutant beta-subunit (Trp8 to Arg8 and Ile15 to Thr15). DESIGN: Clinical study. PATIENTS: We studied 40 healthy non-pregnant Japanese women of known zygosity for the LH beta-subunit gene (3 homozygotes for the mutant gene, 17 heterozygotes, and 20 homozygotes for the wild type). All women had normal ovulatory cycles. MEASUREMENTS: Serum LH status was determined by comparing LH immunoassays results using a monoclonal antibody recognizing only wild-type LH with those from a polyclonal antibody assay recognizing both variant and wild-type LH. The ratio of monoclonal to polyclonal immunoassay results determined the serum LH status. LH secretion in response to a GnRH stimulation test was measured. RESULTS: All women with the wild-type LH showed a normal response of LH to GnRH according to both assays. Over the time course of the response, the ratios in women with wild-type LH showed no remarkable changes. The response curves in women heterozygous for the mutant peaked 15-30min after GnRH injection; their response patterns included a statistically significant decrease in the rates of response at 15min after injection. CONCLUSIONS: There are the differences in circulatory kinetics between the two LH forms and in regulation of the two types of LHbeta genes. The maximal response of the variant LH to pituitary stimulation with GnRH appears to be greater than that of wild-type LH.