Rapid mixing is important in the overall process of coagulation. In-line jet injection blenders are efficient rapid-mix units that are inexpensive to construct and operate. Literature pertinent to the mixing of a side jet is discussed, and analytical relationships describing optimum conditions for rapid mixing of central jets are derived. A previously proposed design procedure for centrally injected jets is refined to include velocity ratio and diameter ratio as design variables. An example problem is included for the rapid-mix step of an alum coagulation system. The authors conclude that both the velocity ratio and the diameter ratio should be considered as design variables in the design of in-line jet injection blenders. Optimum conditions for small diameter ratios are characterized by a constant value for the momentum ratio, whereas the optimum momentum ratio for large diameter ratios is not a constant and depends on the diameter ratio. Includes 26 references, table, figures.