The effect of the zearalenone hydrolase ZenA enzyme, added to the rumen fluid–buffer (RF-B) mixture with low (5.8) or normal (6.8) pH, on the molar β-zearalenol (β-ZEL) concentration, formed out of the parent toxin ZEN, in the in vitro rumen model. The enzyme was tested at two different concentrations (125 µg/mg feed and 62.5 µg/mg feed) during an incubation period of 6 h. The molar β-ZEL concentration is expressed relative to the maximum molar ZEN concentration detected. At the low pH of the RF-B mixture and in all treatments with enzyme addition, β-ZEL accumulation was below the detection limit, resulting in overlapping results.