7S nerve growth factor (7S NGF) and nerve growth factor I (NGFI) are NGF-containing protein complexes isolated from mouse submandibular glands by different protocols, and reports suggest that the molecules differ chemically. In this study, we compared the molecular properties and subunit compositions of the two proteins. Purified 7S NGF and NGFI electrophoresed to identical positions on polyacrylamide gels in nondissociating buffers, with electrophoretic mobilities indistinguishable from that of unpurified NGF in salivary gland extracts. Ultraviolet absorption curves were identical, and sedimentation coefficients were similar (7.3 +/- 0.25 S for 7S NGF; 7.2 +/- 0.2 S for NGFI) as determined by sedimentation velocity analysis. By sedimentation equilibrium analysis, molecular weights of 135 000-140 000 were obtained for both complexes at protein concentrations in the centrifuge cell greater than 85 micrograms/mL; when protein concentrations within the centrifuge cell ranged from approximately 30 to 100 micrograms/mL at equilibrium, both complexes dissociated. Molecular weight values determined by gel filtration on Bio-Gel P300 and Sephadex G200 resins were similar for both proteins, and the values determined on Sephadex agreed with those obtained by ultracentrifugation. The subunit compositions of the complexes were also similar as determined by nonequilibrium isoelectric focusing, NGFI being composed of proteins that migrated to positions identical with those of the alpha, beta, and gamma subunits of 7S NGF. Furthermore, the stoichiometry of the subunits was similar in the two complexes as determined by radioimmunoassays to each of the subunits and by densitometric analysis of electrophoretic gels. Both methods showed that the complexes contain approximately 2 mol of the alpha and gamma subunits per mole of beta-NGF.(ABSTRACT TRUNCATED AT 250 WORDS)