Ferredoxins were isolated and purified from leaves of different species of Nicotiana and Petunia and from spinach leaves. Their spectral properties, degree of homogeneity, and molecular weights were determined. The preparations were further analyzed by polyacrylamide gel electrophoresis of tryptic hydrolysates. This allowed us to distinguish between not only ferredoxins of Nicotiana, Petunia, and spinach, but even ferredoxins of various Nicotiana species. We used the differences in tryptic peptide compositions as phenotypic markers to study the mode of inheritance of chloroplast ferredoxin to see whether the coding site is in the chloroplast or in the nucleus. Analysis of the tryptic peptide composition of ferredoxin from different interspecific hybrids of Nicotiana showed that the characteristics of both parental ferredoxins were present. The results indicate that the primary structure of at least the male ferredoxin is coded for in the nucleus. In some of the hybrids the relative contribution of the male parent appeared to be low, suggesting that the female genome (presumably that part located in the plastome) exerted a dominating influence.