Abstract: The invention relates to a system for detecting at least one species present in a fluid, preferably for detecting at least one circulating cell or aggregate of cells present in a human or animal biological fluid, and in particular circulating tumour cells (CTC) present in a blood fluid, the detection system comprising means (20) for filtering the fluid, the filtering means (20) comprising a filtering membrane (21), the filtrating membrane comprising at least one pore (22) designed to retain a species of a given type present in the fluid, the filtration means (20) further comprising at least one opening (23) designed to ensure, during operation within the fluid, the continuous circulation of the biological fluid, even when the at least one pore (22) is occupied.