Class a and class b 30 to 40S RNA subunits obtained by heat dissociation from the 60 to 70S RNA complex of avian tumor viruses were compared with several RNA standards by electrophoresis in formamide-polyacrylamide gels. Class a RNA was found to have a lower electrophoretic mobility and hence probably a higher molecular weight than class b RNA. The absolute molecular weight of class a and b RNA could not be determined with accuracy, because the relationship between logarithm of molecular weight and mobility of the RNA standards was not linear. The size of class a RNA fell into the range of 2.4 x 10(6) to 3.4 x 10(6) daltons and that of class b into the range of 2.2 x 10(6) to 2.9 x 10(6) daltons, depending on the standards used. The possible biological significance of this difference is discussed.