The invention relates to a method for mixing at least two fluids using an externally mixing nozzle for medical purposes, which has at least two outlet channels (10, 20) and at least two inlet openings (13, 23) with different or identical cross-sections, wherein two fluids with different volumetric flows and/or different viscosity are sprayed, and wherein the ratio of the cross-sections of the inlet channels (13, 23) and/or the outlet channels (10, 20) corresponds to the ratio of the volumetric flows so that the fluids flow with substantially identical flow speeds through the outlet channels (10, 20) and/or the inlet openings (13, 23). The invention furthermore relates to an externally mixing nozzle, a medical instrument and a medical device for spraying substances, in particular biological material.