Radioreceptor assay and thermostability test for the androgen receptor in two cases with complete testicular feminization syndrome were performed in regard to the fibroblasts cultured from genital skin on the basis of dispersed whole cell binding assay (Eil et al., 1980). No [³H]dihydrotestosterone binding to the androgen receptor was observed in case 1 (receptor negative), while maximum binding capacity and dissociation constant of androgen receptor for [³H]dihydrotestosterone in case 2 were 21000 sites per cell and 1.67×10^-10M (receptor positive). The specific binding of [³H]dihydrotestosterone to the androgen receptor in case 2 decreased remarkably to 6.6% after high temperature (42°C) incubation in comparison with that at 22°C incubation. The specific binding of [³H]dihydrotestosterone to the androgen receptor in normal controls decreased down to 81.6% at high temperature incubation. Thermostability test was useful to demonstrate qualitative abnormality of androgen receptor in receptor positive testicular feminization syndrome.