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Injection locking of a GaAlAs laser by coupling light into a nonlasing Fabry-Perot mode separated by up to 22 mode spacings (60 Å) from the free-running lasing mode is demonstrated. The gain spectra for operation above threshold are determined from measurements of the wavelength dependence of the injected power required for locking. These results are compared to gain curves deduced from spontaneous emission spectra measured slightly below threshold. For the greatest wavelength separation, the gain difference was found to be about 4 cm-1, with injected power levels of 1 percent of the total slave laser power required to obtain 75 percent of the total energy at the master laser frequency.