The specificity and sensitivity of a fluorescent antibody technique applied to growing microcolonies has been investigated using serotypes ofEscherichia coliresponsible for infective enteritis as a model. Microcolonies of 10E.coliserotypes showed bright fluorescence when treated with homologous conjugated antiserum but no fluorescence when treated with heterologous conjugated antisera. Microcolonies of Enterobacteriaceae strains of other genera or ofE.colistrains not associated with infective enteritis showed no fluorescence when treated with conjugated antisera prepared against the enteritis serotypes.Experiments with artificially infected faecal suspensions showed that the sensitivity of the microcolony technique was approximately one hundred times greater than that of the direct smear method. A number of other advantages and possible disadvantages of the microcolony technique are discussed and its usefulness in epidemiological work is suggested.