A number of factors governing the polymerization of α-urease, the monomer which represents the traditional enzyme of 480,000 molecular weight, have been explicitly defined. Of major importance is mercaptoethanol (or other mercaptans), the presence of which was essential to the regular preparation of pure α-urease, and the removal of which, by gel filtration or by dialysis, regularly resulted in the formation of active polymers over the subsequent days to weeks. The polymerization proceeded several times faster at 25 ° than at 4 °, and complete separation of the trimer, the dimer, and the monomer demonstrated that the n-mers are isolable entities. The monomer and dimer so isolated also produced cross-inhibition when mixed together in assay solutions. Upon addition of mercaptan the polymers could be converted back to pure α-urease, thus demonstrating that the latter is indeed the polymerizing unit and that intermolecular disulfide bond formation is involved in polymerization. Titration of neutral urease solutions to pH 5.0 resulted in the conversion of about half the α-urease into higher polymers, which survived the return of pH to 8.5. Titration to pH 4.0 resulted in the splitting of some α-urease into active half-units (which again survived the return of pH), as well as a moderate loss in total activity. At neutral pH the polymerization of urease was concentration dependent, being roughly first order; and the kinetic pattern at 25 ° was similar to the theoretic Flory distribution for simple bifunctional condensations, compatible with the view that the enzyme polymers are linear aggregates.