The molecular weight of 7 S nerve growth factor has been studied in the analytical ultracentrifuge between pH values 6.8 and 7.8. At pH 6.8, where no dissociation is observed, the molecular weight was found to be 137,000 plus and minus 7,000. Between pH values 7.4 and 7.8 there is some dissociation. Using the data from this study and results in the literature, a model of 7 S nerve growth factor, (alpha beta gamma)2, in reversible equilibrium with a subunit complex, (alpha beta gamma), is proposed.