About 70% of the protein in isolated Bacillus subtilis spore coats was solubilized by treatment with a combination of reducing and denaturing agents at alkaline pH. The residue, consisting primarily of protein, was insoluble in a variety of reagents. The soluble proteins were resolved into at least seven bands by sodium dodecyl sulfate gel electrophoresis. About one-half of the total was four proteins of 8,000 to 12,000 daltons. These were relatively tyrosine rich, and one was a glycoprotein. There was also a cluster of proteins of about 40,000 daltons and two or three in the 20,000- to 25,000-dalton range. The insoluble fraction had an amino acid composition and N-terminal pattern of amino acids very similar to those of the soluble coat proteins. A major difference was the presence of considerable dityrosine in performic acid-oxidized preparations of insoluble coats. Coat antigen including a 60,000-dalton protein not present in extracts of mature spores was detected in extracts of sporulating cells by immunoprecipitation. This large antigen turned over in a pulse-chase experiment. Antibodies to either the array of 8,000- to 12,000-dalton coat polypeptides or to the larger coat proteins reacted with this 60,000-dalton species, suggesting a common precursor for many of the mature coat polypeptides. Spore coats seem to be assembled by processing of proteins and by secondary modifications including perhaps dityrosine formation for cross-linking.