The magnetization curve of the ferromagnetic oxides (LaPb)MnO3 can be fitted to a molecular field model if a quartic term in the magnetization is introduced into the magnetic exchange energy. A search is made for volume or distortion anomalies as one of the compounds, La0.62Pbo0.38MnO3, is heated through its Curie temperature. No anomalies sufficiently large to explain the magnitude of the quartic term are found and it is therefore concluded that the quartic term does not arise because of a volume-or distortion-dependent magnetic exchange energy.