SUMMARYThe practical application of a simple, reproducible androgen receptor assay using dispersed fibroblasts is described for the investigation of patients with the androgen insensitivity syndrome (AIS). The concentration of androgen receptors (Bmax) in genital skin fibroblasts derived from normal subjects is 814 ± 168 ×−18moles/μg DNA (mean ± SD) and the binding affinity (Kd) is 0·91 ± 0·26 × 10±10M at 37°C, using [3H]‐5α‐dihydrotestosterone (DHT) as the labelled ligand. Studies in patients with phenotypic signs of complete or partial AIS showed either absent (receptor‐negative), decreased (receptor‐deficient) or normal/increased (receptor‐positive) specific binding of DHT to the receptor. In some mutant cell lines, there was evidence of thermolability and increased rate constant of dissociation of the androgen‐receptor complex, suggesting a possible structural abnormality of the receptor protein. This simple receptor assay can be used to delineate quantitative and qualitative defects of the androgen receptor in a significant number of patients with a wide spectrum of phenotypic abnormalities associated