Plasma and fibroblast cellular fibronectins from three different species were compared for structural similarities and differences. Partial tryptic digestion of either human or chicken plasma and cellular fibronectins yields homologous protease-resistant domains within a species but few homologies between species regardless of the source. Within a species, human or chicken plasma and fibroblast cellular fibronectins are immunologically indistinguishable as determined by the ELISA technique. There is limited immunological cross-reactivity between species. Two-dimensional tryptic peptide maps of fibroblast cellular and plasma fibronectins from the same species are also very similar: 85-95% of the spots on such maps comigrate. When peptide maps from different species are compared, no more than 10% of the spots comigrate. Three models for the genetic origin of cellular and plasma fibronectins in vertebrates are considered. A model in which both fibroblast cellular and plasma fibronectins arise from the same gene is the simplest that is consistent with the data.