Manure additives are widely used to mitigate gas and odor emissions from manure or improve manure properties. However, the reported effectiveness of some manure additive products has been mixed and most of the studies on poultry manure have been on chemical additives. A laboratory study was conducted to evaluate an enzyme-based commercial manure additive for its potential reductions of ammonia (NH3), carbon dioxide (CO2), hydrogen sulfide (H2S), and odor releases from layer hen manure. Eight 122-cm tall and 38-cm diameter reactors, four treated with the additive and four control, were studied for 38 days with manure from commercial layer hen houses. The reactors were initially filled with 66-cm height manure followed by weekly additions of 5 cm each. Ventilation air was supplied to the reactor headspace to simulate winter ventilation rates in layer hen houses. Concentrations of NH3, CO2, and H2S in the reactor exhaust air were measured with gas analyzers for 10 minutes, six times daily. Odor intensity was assessed by a trained odor panel. Open-headspace tests were also conducted to corroborate the observations in the reactor study. Study results showed that the average 4-reactor group mean release rates ± standard deviations of NH3 were 17.5 ± 14.3 and 20.1 ± 12.6 µg s–1 from the control and treated groups, respectively. Those of CO2 were 1091 ± 149 µg s–1 from the control and 1143 ± 217 µg s–1 from the treated groups. Release of H2S from the reactors could not be detected. The odor intensities were 3.5 ± 0.3 and 3.4 ± 0.3 before and after the additive spray, respectively. Application of the additive onto the manure did not demonstrate an effect on the releases of NH3 (P = 0.41), CO2 (P > 0.23), and odor (P > 0.71).