For purifying carboxysomes of Thiobacillus neapolitanus an isolation procedure was developed which resulted in carboxysomes free from whole cells, protoplasts and cell fragments. These purified carboxysomes are composed of 8 proteins and at the most of 13 polypeptides. The two most abundant proteins which make up more than 60% of the carboxysomes, are ribulose-1,5-bisphosphate carboxylase and a glycoprotein with a molecular weight of 54,000. The shell of the carboxysomes consists of four glycoproteins, one also with a molecular weight of 54,000. The other proteins are present in minor quantities. Ribulose-1,5-bisphosphate carboxylase is the only enzyme which could be detected in the carboxysomes and 3-phosphoglycerate was the only product formed during incubation with ribulose-1,5-diphosphate and bicarbonate. The supernatant of a broken and centrifuged carboxysome suspension contained the large subunit of ribulose-1,5-bisphosphate carboxylase. The small subunit of ribulose-1,5-bisphosphate carboxylase was found in the pellet together with the shell proteins which indicates that the small subunit of ribulose-1,5-bisphosphate carboxylase is connected to the shell.