A method for the simultaneous detection of several antigens or antibodies in a fluid makes use of a mixture of particles which contains populations of particles which can be distinguished from one another in the following way: Each population has a combination, specific for it, of the following features: 1) fluorescent substances with different emission spectra, 2) quantity of these fluorescent substances, 3) particle size. Each particle population is furthermore loaded with a different type of antibodies or antigens. Use of this particle mixture makes it possible to carry out simultaneous investigation for several types of antigens or antibodies in a time- and effort-saving manner. The particle mixture is mixed with the fluid containing the antibodies or antigens to be detected. The subsequent reaction steps correspond to the steps in a conventional immunofluorescence method.; Finally, a measuring device (e.g. flow cytometer) is used to measure the fluorescence (emission spectrum and intensity) and size of each particle. A computer identifies the particle on the basis of the measured data and assigns the measured immunofluorescence to a defined specificity.