A simple coaxial jet mixer for rapid and efficient confluent mixing under laminar flow conditions (Re < 5) is described. This device demonstrates exceptional control of mixing between two laminar streams by creating shear forces due to variable flow velocities at the point of confluence. It is suitable for flow injection and cytometric analyses of rapid kinetic events which require contact mixing of two solutions and subsecond measurements of the evolving reaction. This apparatus was devised for flow injection cytometry as performed on a Becton Dickinson FACS Analyzer. Under normal cytometric conditions and at a sample introduction rate of 60 microL/min, the laminar jet mixer is capable of complete mixing of two solutions within 55 ms. Kinetic measurements can be performed on the FACS Analyzer in a variable time range of 100 ms to 3 min with 14-30 ms temporal resolution of the studied event. Since no boost in core flow is required, potential spectral distortions due to core flow variations are eliminated. This coaxial jet mixer can be easily constructed and employed on a variety of cytometers as well as conventional flow injection analysis systems, since it is an effective mixer under most flow conditions.