Synthetic spectra have been computed for comparison with the spectrum of the slow supernova of 1961 in NGC 1058. Apparently redshifted emission lines in the spectrum of the supernova are largely caused by strong violet-shifted absorption components. The helium-to-hydrogen ratio appears to have been several times higher than in the Orion Nebula. No significant abundance peculiarities among the metals (scandium through nickel) have been detected. The chemical composition of the matter ejected from SN 1058 may have been similar to the composition of the Sun except for a deficiency of hydrogen. Consideration of simple models of the geometry and velocity distribution in the envelope of the supernova leads to lower limits to the mass of the observable emitting and absorbing regions exceeding 0.3 .