We have adapted the MTT-ESTA bioassay for human GH (hGH) to measure the lactogenic bioactivity of the hormone in human serum. This highly quantitative in vitro colorimetric bioassay is based upon the reduction of a tetrazolium salt, 3-[4,5-dimethyl-thiazol-2-yl]2,5-diphenyl tetrazolium bromide (MTT), to its formazan by lactogen-activated Nb2 cells. Relatively high concentrations of human serum (1-10%) modified responses to the hormone in a complex manner. As the serum effects varied between samples, it proved impossible to adapt the bioassay by the conventional approach of using a lactogen-depleted serum as a representative matrix. However, as the Nb2 cells were exceptionally sensitive to hGH, the serum effects could be diluted out. We adopted a dilution strategy by which all samples of human serum were included in the bioassay at a concentration of 0.625% or less. A valid assay was obtained, as judged by the criteria of parallelism between diluted samples and hGH standards, and recoveries of spiked samples that were close to 100%. Hormonal specificity was achieved with the use of a highly specific anti-PRL antiserum. A within-assay precision of between 2-5% over the dose range of 0.03-0.96 microgram hGH/L was attained. As only highly diluted samples could be used, the sensitivity of the clinical bioassay was 1.2-2.4 micrograms hGH/L. The between-assay precision was estimated to be 11% and 9% at initial hGH concentrations in serum of 4.8 and 19.2 micrograms hGH/L, respectively. By exploiting the high sample capacity of the eluted stain bioassay system, we followed the changes in bioactivity and immunoactivity of hGH in multiple timed samples after stimulation of hGH secretion in an adult by GHRH. Systematic and progressive changes were observed in the bioactive/immunoactive ratios. Analogous changes were observed after insulin-induced hypoglycemia in a child with short stature. We speculate that the changes in the bioactive/immunoactive ratios reflect alterations in the proportions of the isoforms of hGH in the circulation after acute stimulation.