The effect of changes in sample isoferritin composition on the behaviour of ferritin 2-site immunoradiometric assays was investigated using a computer model and experimental studies. Modelling studies predicted that under conditions where a sample has a different isoferritin composition from the assay standards, progressively higher values will be generated for the apparent ferritin content of the sample when it is analysed at increasing dilutions. In addition, the assay results will underestimate the actual ferritin content of such samples at all dilutions. Dilutional discrepancies in assay results and underestimation of sample ferritin concentrations were found in practical assays when synthetic samples with demonstrably different isoferritin profiles were analysed. Differences in isoferritin composition between assay standards and assayed samples may therefore be a cause of dilutional discrepancies in some results from ferritin 2-site immunoradiometric assays.