<latex>$\delta$</latex>-Aminolaevulinic acid dehydratase catalyses the synthesis of porphobilinogen. The enzyme has a molecular mass of 285 000 and is composed of eight similar subunits of molecular mass 35 000. The N-terminal amino acid is acylated, and the number of peptides found on tryptic digestion equals the number of lysine and arginine residues per mass of 35 000. The eight subunits are apparently arranged at the corners of a cube and therefore have dihedral (D<latex>$_4$</latex>) symmetry. The bovine liver enzyme which has been crystallized contains 4-6 atoms of zinc per mole of enzyme. The apo-enzyme obtained on prolonged hydrolysis can be reactivated by the addition of zinc or cadmium ions. The dialysed enzyme must be first treated with dithiothreitol. There are two very active SH groups in a total of 6-7-SH groups per subunit. The substrate forms a Schiff base with the <latex>$\epsilon$</latex>-amino group of a lysine residue. Reduction of the Schiff base with NaBH<latex>$_4$</latex> should reveal the number of active sites per mole of enzyme. It appears that only four of the eight subunits form a Schiff base with the substrate indicating that the enzyme exhibits the phenomenon of either half-site reactivity or negative cooperativity. The enzyme appears to have a strong subunit-subunit interaction for an immobilized preparation remained stable for at least a month. An immobilized enzyme preparation was treated in a manner so that it dissociated into tetramers. Both the eluate and the protein still attached to the Sepharose on a column were enzymically active. The bound enzyme could not reassociate under assay conditions but still contained about 50% of the original enzyme activity. It would seem that the enzyme is active when composed with less than eight subunits.