A method of determining the concentration, relative activity or effects of a biologically active substance in a sample based on the amount of radioactive thymidine, e.g., H3-thymidine, taken up by a culture of reporter cells (cells which depend for their viability and/or reproduction on the substance of interest) following application of the sample. The amount or activity of that substance in the sample is calculated by measuring the incorporation of radioactivity by the text cell culture following application of the sample compared with the incorporation of radioactivity following application of a reference standard.