In response to a need expressed by many workers for a means of measuring immunofluorescence emission from single cells, we have devised a method of measuring such emission, in terms of international physical units, from single bacteria labelled with fluorescent antibody. The equipment, which incorporates fibre optics, has been calibrated in relation to a standard light source calibrated by the National Physical Laboratory, Teddington, Middlesex, England, so that photometric measurements of fluorescence emission may be expressed in candelas per square metre. Uranium glass used as a transfer standard remained sufficiently stable over a period of at least 18 months. The effect of fading of fluorescence on reproducibility of measurements has been largely overcome. Background illumination and flare from nearby fluorescing bacteria did not have a significant effect. Measurements obtained with varying dilutions of an Escherichia coli 026:B6 antibody conjugated with fluorescein isothiocyanate have been evaluated statistically. Repeatability was of a high order. By means of a conversion factor, the equation of the regression line for the relation of candelas per square metre to dilution values may be used to relate fluorescence emission to dilution values over the range examined thereby improving the prospects for standardization in immunofluorescence techniques.