Five commercial RIA kits designed to measure human insulin concentrations were compared for their ability to measure feline insulin. Kit evaluation included dilutional parallelism for specificity, sensitivity and intra- and interassay precision. Three kits (RSL, Diagnostic Products and Pharmacia) performed well and can be recommended for use in cats. Two kits (Corning and Amersham) had poor specificity and precision and are not suitable for use in cats.