Hemicell® HT is a feed additive with endo-1,4-β-d-mannanase as the main enzymatic activity that is available in solid (HT) and liquid (HT-L) forms. The production strain of the enzyme is a genetically modified strain of Paenibacillus lentus. The recipient strain is considered to be safe, the sequences introduced to obtain the production strain do not raise safety concerns. The absence of the production strain and of recombinant DNA in the product was not proven. The additive is safe for the target species at the corresponding recommended doses. The use of Hemicell® HT as a feed additive does not give rise to concerns for consumers. Hemicell® HT and Hemicell® HT-L are not irritant to the skin and eyes; Hemicell® HT is a skin sensitiser. No specific data were provided on the effects on the respiratory system; however, considering the nature of the active substance, the additive is considered a potential respiratory sensitiser. The active substance of the additive is a protein, and as such, it will be degraded/inactivated during passage through the digestive tract of the animals or in the environment. However, uncertainty remains on the absence of the production strain and of recombinant DNA in the product, consequently, the EFSA FEEDAP Panel could not conclude on the environmental safety of the product with regard to the genetically modified production strain. The additive has the potential to be efficacious in chickens for fattening at 32,000 U/kg feed and at 48,000 U/kg feed in turkeys for fattening and weaned piglets. These conclusions were extended to chickens reared for laying and turkeys reared for breeding and extrapolated to minor poultry species for fattening or reared for laying/breeding. The Panel could not conclude on the efficacy of the product in pigs for fattening or in minor porcine species.