The invention relates to a method for detecting biological material in an airstream, in which method the airstream is fed with the aid of sampling devices, a light beam is emitted towards the airstream, a fluorescence signal depicting the fluorescence of the particle is created, a scattering signal depicting the scattering of the light of the particle is created, the fluorescence signal and the scattering signal are converted into discrete values, and an alarm value is defined. The discrete values are recorded cumulatively as hit points in an at least 2-dimensional measurement space equipped with selected dimensions, at least one index area is preselected from the measurement space, a cumulative index is calculated at an index frequency from the hit points accumulated on the each preselected index area, and an alarm value, showing the presence of a selected biological material, is defined from the indices by using a preselected criterion.