A study of the threshold of stimulated emission for optically pumped PbSe and PbTe is presented. The influence of external parameters such as magnetic field and misfit strain on the threshold as well as the differential quantum efficiency of stimulated emission are analyzed. It is shown that effective energy gap shifts caused by the external parameters only weakly influence the threshold, whereas the main effect arises from the redistribution of the carriers in the multivalley band structure when the degeneracy of the carriers in the valleys at theLpoints of the Brillouin zone is lifted. A consistent qualitative theoretical explanation for the effects is given. ©1995 American Institute of Physics.