Salmonella bacteria were stained with serial dilutions of anti‐Salmonella conjugates of different F/P ratios and the staining intensity was measured quantitatively in an ultramicrofluorometer. In any given dilution of the conjugates, a stronger fluorescence was obtained with the more highly labelled conjugates. The dependence of fluorescence intensity on F/P ratio varied with the dilution of the conjugate. Similar results were obtained by the indirect immunofluorescence method. In four bacterial systems the direct and indirect immunofluorescent staining methods were compared quantitatively. The indirect method was 5 to 30 times more sensitive than the direct comparing the last dilutions giving a positive reaction by visual observations. The standard deviation of the intensity values of the stained bacterial cells was between 10 and 40 per cent of the mean. Different sources of variation in the quantitative measurement technique are discusse