The results of this investigation indicate that neurofilaments of mammals (human and rabbit) are composed of four protofilaments each of which is formed of globular units connected by longitudinal bars. A cross-view of neurofilaments reveals the presence of four globular units, 20-25 A in diameter, connected by four transverse bars, 25-30 A in length. Two of the transverse bars appear more electron-dense (darker) than the remaining two. A longitudinal view of neurofilaments shows two protofilaments bonded by the darker of the transverse bars to form a two-stranded band. The remaining two protofilaments form a band in a similar manner. The two two-stranded bands, wrapped helically, form a four-stranded neurofilament. The axial periodicity between twists along each neurofilament ranges from 200 A to 270 A. A three-dimensional diagram of neurofilament structure is presented.