In 195 European bisons divided into four groups (Group 1, 0-3 year-old males; Group 2, 0-3 year-old females; Group 3, mature bulls, over 3 years old; Group 4, mature cows, over 3 years old) seasonal changes in red blood cell number and diameter, haemoglobin level, haematocrit value, index F, MCH, MCHC and MCV were studied. 2. Seasonal cyclicity was found only in red blood cell diameters in all groups. 3. In young males the cyclicity was found in MCHC and in MCV only. 4. In young females the cyclicity was found in Hb and Hct values, in MCH and in MCV. 5. In mature bulls cyclicity was found only in RBC diameter and in MCV. 6. In mature cows the cyclicity was found in index F, MCH and in MCHC values. 7. Seven out of 14 acrophases of cyclic indices occurred just before autumnal equinox and three before vernal equinox.