The overall efficiency of a thermonuclear reactor depends markedly on the efficiency of neutral-beam injection. The model of the injector system is based on direct conversion of the unneutralized portions of the ion beam. The injection efficiency is estimated from assumptions on the performances of the components of the injection system. Based on this model, deuterium or tritium atoms derived from negative ions and between 100 keV and 1000 keV in energy can be injected into the reactor with an efficiency of 80 to 90%. Without direct conversion this efficiency will drop to 65 to 85%. For energies around 100 keV, D+ ions can be used to obtain a system efficiency of about 75%, which is higher than can be obtained with ions. The same model predicts 80% efficiency below 200 keV for injection of 3He atoms derived from positive ions, and less than 70% efficiency between 200 keV and 700 keV using 3H− ions.