Enzyme-linked irnmunosorbent assay (ELISA) provided a rapid and extremely sensitive technique for the detection and quantification of Melissococcus pili/oil in honeybee (Apis mellifera) adults and larvae. The detection threshold for M. pIli/oil cells in homogenates of adults and larvae was found to be approximately 105 cells per ml, two orders of magnitude more sensitive than microscopical techniques previously used. Samples of larvae, nurse bees and foragers from an apparently healthy colony were tested by the ELISA procedure and in some cases were found to contain up to 5 X 107 M. plutoll cells per insect. These results were confirmed by haemacytometer and plate counts, and substantiate the ability of M. plutoll to establish enzootic infections in apparently healthy colonies.