The effects of the injection of a light, medium, and heavy gas on the skin friction and heat transfer characteristics of the laminar boundary layer flow over a flat plate were investigated. This was done by a numerical solution of the governing differential equations for injection rate parameters of -0, -0. 1, -0.2; Mach numers from 0 to 3; and wall-to-stream temperature ratios of 0.25, 0.50, and 1.00. The results show that the selection of discrete injection systems must represent a favorable compromise between conflicting factors (such as small molecular weight and large molecular diameters) which influence the amount of reduction in skin friction and heat transfer desired.