The aim of the present study was to investigate the effect of cyclophosphamide on selected parameters of humoral and cellular immunity in rats. The experimental material comprised 48 adult Wistar rats aged 14 weeks, including 24 females and 24 males. The animals were initially divided into two groups: control (K) and experimental (C), each comprising 12 males and 12 females. Over a period of three consecutive days (days 1-3), 24 experimental group rats were administered cyclophosphamide intramuscularly at a rate of 50 mg/kg body weight/day. At the beginning of the experiment (day 0) and on the 8th, 15th and 22nd day of the study, six control group (K) rats (three males and three females) and six experimental group (C) rats (three males and three females) were sacrificed. Arterial blood samples were collected and diluted with heparin to determine and compare selected parameters of humoral and cellular immunity, including total protein levels, -globulin levels, lysozyme activity, ceruloplasmin activity, proliferative response of blood lymphocytes (MTT) after stimulation with LPS or ConA, the metabolic activity (RBA and Bursttest) and potential killing activity (PKA and Phatogest) of phagocytes. The results of the study indicate that cyclophosphamide had a significant immunosuppressive effect on all of the investigated parameters of humoral and cellular immunity in rats.