A method for analyzing a biological sample by means of an analysis instrument, the biological sample comprising biological agents and being arranged in an analysis receptacle in a field of view of a holographic imaging system, the method comprising: - acquiring a holographic image of the biological sample at an instant of measurement, the holographic image associating an intensity value with each pixel of said holographic image, the intensity values lying within a range of intensities extending between a minimum intensity value and a maximum intensity value, characterized in that - determining, from the holographic image acquired, an image mask which associates an active or inactive state with each pixel of the holographic image in accordance with the intensity values of the pixels of the holographic image, so that an inactive state is associated with pixels of the holographic image which correspond to artefacts caused by elements present in the field of view other than biological agents, the image mask being determined so that the inactive state is associated with at least some pixels of a connected set of pixels having intensity values within a masking range forming a subset of the intensity range, said masking range being delimited by at least one masking threshold, the determination of the image mask comprising the comparison of intensity values of the pixels of the holographic image with the at least one masking threshold and the association with each pixel of a first mask value indicating the inactive state of said pixel or a second mask value indicating the active state of said pixel in accordance with the result of the comparison, - determining a value of at least one biomass parameter representative of the quantitative spatial distribution of biological agents in the field of view, on the basis of only those pixels of the holographic image that exhibit an active state defined by the image mask, and - supplying said value of the biomass parameter among the analysis results. - acquiring a holographic image of the biological sample at an instant of measurement, the holographic image associating an intensity value with each pixel of said holographic image, the intensity values lying within a range of intensities extending between a minimum intensity value and a maximum intensity value, characterized in that - determining, from the holographic image acquired, an image mask which associates an active or inactive state with each pixel of the holographic image in accordance with the intensity values of the pixels of the holographic image, so that an inactive state is associated with pixels of the holographic image which correspond to artefacts caused by elements present in the field of view other than biological agents, the image mask being determined so that the inactive state is associated with at least some pixels of a connected set of pixels having intensity values within a masking range forming a subset of the intensity range, said masking range being delimited by at least one masking threshold, the determination of the image mask comprising the comparison of intensity values of the pixels of the holographic image with the at least one masking threshold and the association with each pixel of a first mask value indicating the inactive state of said pixel or a second mask value indicating the active state of said pixel in accordance with the result of the comparison, - determining a value of at least one biomass parameter representative of the quantitative spatial distribution of biological agents in the field of view, on the basis of only those pixels of the holographic image that exhibit an active state defined by the image mask, and - supplying said value of the biomass parameter among the analysis results.