The mature forms of two recombinant murine interferons alpha, alpha 1 and alpha 4, have been expressed in vitro using an established transcription and translation system. The relative specific antiviral activity, antiproliferative activity and the natural killer cell stimulating activity of both subtypes were compared in vitro. While the antiviral and natural killer cell stimulating activities of the 2 subtypes were similar, the relative antiproliferative activities varied markedly. On the basis of equal molar inputs, MuIFN-alpha1 had less than 8% of the antiproliferative activity of MuIFN-alpha4. This data shows that a large functional difference exists between these two subtypes which are known to be expressed at different levels in mouse L-cells in vitro.