Disclosed is a method for quantifying living organisms (1, 4) in a liquid sample (2), the method comprising the steps of •guiding the liquid sample (2) into a chamber (3), •analysing pictures of the sample (2) inside the chamber (3) to detect the number of organisms (4) moving by themselves in the sample, •illuminating the sample (2) with light in at least a part of the violet-blue spectrum while detecting the number of organisms (1) that are fluorescent in the sample inside the chamber (3), and •analysing pictures of the sample (2) inside the chamber (3) while illuminating the sample (2) with light in at least a part of the violet-blue spectrum to detect the number of organisms (1, 4) that are both moving by themselves and fluorescent. A device (15) for quantifying living organisms (1, 4) and use of a device (15) is also disclosed.