${eta}-Lactamase$ was isolated from the culture filtrate of the penicillin producing strain, Penicillium chrysogenum Q176. When the pH of the medium was adjusted to 5.0 at the start of culture, a rapid increase in pH accompanied by the synthesis of penicillin was observed in the first $2{sim}4$ days. When the pH of medium was brought to 6.0 or 7.0 the opposite was observed: high yield of the enzyme and little of the antibiotics in the medium. The optimum enzyme activity was at a temperature of $40^{circ}C$ and around pH 7.0. A partially purified enzyme was assayed on several different substrates including penicillins V and G, 6-aminopenicillanic acid, cephalosporin C. The V max values calculated were 24.5, 20.4, 7.6, and 6.1 mmoles/hour, and the $K_m$, values were 16.4, 12.6, 7.5, and 6.9 mM in the order given.
