Turkey histomonosis poses a serious threat to poultry production due to the ban on the use of effective drugs. The aim of the study was to evaluate the influence of a phytoncidal feed supplement on the course of histomonosis. The preparation was also analysed for immunomodulatory properties. Clinical observations and production monitoring were conducted in a flock of turkeys with histomonosis from their 11ᵗʰ to 56ᵗʰ weeks of life which were treated with the adiCoxSᴼᴸPF soluble supplement in a dose of 2.5 mL/L water. Later the preparation was used in a preventive dose (1 mL/L). The influence on the immune system was evaluated in broiler turkeys having been given adiCoxSᴼᴸPF for 3 days in doses of 1 or 3 mL/L. The T and B lymphocyte percentages in turkey blood and spleen tissue were analysed with flow cytometry. ELISA was implemented to evaluate antibody titres after Ornithobacterium rhinotracheale vaccination, and biochemical analyses were performed to evaluate the supplement’s safety. AdiCoxSᴼᴸPF was found effective in therapy and prevention of histomonosis. Additionally, adiCoxSᴼᴸPF stimulated both humoral and cell-mediated immune mechanisms, without impairing the functions of internal organs. The treated turkeys also yielded better production results (eggs/hen, fertility, and hatchability). AdiCoxSᴼᴸPF possesses immunomodulatory properties and it can be used successfully in the prevention and therapy of histomonosis in turkeys.