Four methods for estimating plasma pepsinogen concentrations were compared to determine the most suitable for routine use. Three of the methods were colorimetric assays which used glycine-buffered albumin, albumin, or haemoglobin as their substrates. The fourth method was a radial diffusion assay which used casein in agarose as its substrate. All four methods gave reproducible results when replicates were assayed on the same day. However, the first method had the least day-to-day variation and was the only method which gave a linear relationship with different concentrations of pepsinogen in a plasma pool.