We apply the multimode semiclassical laser theory to the problem of line narrowing and frequency selection in an injection locked laser. Our goal is to predict from the spectroscopic properties of a given active medium, the behavior of a broad-band laser that is under the influence of a narrow-band injected signal. Analytic expressions for the injected signal strength necessary for single-mode operation are obtained for both homogeneously and inhomogeneously broadened lasers, and for small-signal and strong-signal regimes. We found that under certain experimental conditions, an injection locked inhomogeneously broadened laser behaves significantly different from a homogeneously broadened lasel.