We analyzed the ability of the BaF3 cell line bioassay to select patients with biologically inactive GH. We first evaluated the biological response of the Ba/F3-hGHR cells to rhGH additional doses from 10 to 5000 pg/ml. The concentration points corresponding to the linear part of the curve were selected. We then analyzed a group of sera, diluted like the standard, including the entire range of GH concentrations that can be analyzed by bioassay. The serum/standard area below the curve ratio was calculated. Serum GH immunoactivity determined by IMMULITE/GH bioactivity ratios was calculated. Our experimental data showed that GH-bioactivity/GH-immunoactivity ratios below 0.303 are indicative of a bioinactive GH molecule. This bioassay would recognize only extreme cases of GH bioinactivity, and it would not be a useful tool in the search for patients with altered forms of GH.