Virulent and certain avirulent strains of BaciUus anthracis harbor a plasmid, designated pXO2, which is involved in the synthesis of capsules. Two classes of rough, noncapsulated (Cap-) variants were isolated from the capsule-producing (Cap') Pasteur vaccine strains ATCC 6602 and ATCC 4229. One class was cured of pXO2, and the other class still carried it. Reversion to Cap' was demonstrable only in rough variants which had retained pXO2. Proof that pXO2 is involved in capsule synthesis came from experiments in which the plasmid was transferred by CP-51-mediated transduction and by a mating system in which plasmid transfer is mediated by a BaciUus thuringiensis fertility plasmid, pX012. Cells of Bacillus cereus and a previously noncapsulated (pXO2-) strain of B. anthracis produced capsules after the acquisition of pXO2. R. A. Packer Colorado Virulent, Cap' Tox+ pXO1, pXO2 A. McChesney NH Virulent, Cap' Tox+ pXO1, pXO2 USAMRII)d Vollum 1B Virulent, Cap' Tox+ pXO1, pXO2 USAMRIID Vollum lB-1 Avirulent, Cap+ Tox-pXO2 This study Vollum 1B VNR-1 Avirulent, Cap-Tox+ pXO0 This study Weybridge (Sterne) Avirulent, Cap-Tox+ pXOl MREe Weybridge UM44 Ind-pXOl UVf of Weybridge Weybridge A Colony variant of Weybridge pXOl C. B. Thorne Weybridge A UM23C1 Ura-, cured of pXO1 None C. B. Thorne Weybridge A UM23C1 tdlO Ura-Cap' pXO2 This study 4229 (Pasteur)