The aim of this study is to compare the performance of three commonly used insulin assays with respect to the detection of exogenous human and porcine insulin added to human or rat plasma. The DPC Immulite human insulin assay, the Mercodia rat insulin enzyme-linked immunosorbent assay and the Linco rat insulin radioimmunoassay were tested. The mean cross-reactivity of exogenous insulin ranged from 25% to 92%. The mean cross-reactivity of Actrapid in human plasma on the DPC Immulite was 56% and was independent of the endogenous insulin concentration. The measurement of exogenous insulin varies according to the source of exogenous insulin, matrix and insulin assay.