SummaryIt appears from this Q.C. survey that automatic cel counting generally gives good results for erythrocytes, but somewhat less good results for white blood cells.Thrombocytic numeration however, gives unreliable and uncomparable results in a Q.C. survey, regardless of the method used, because of the extreme sensitivity of the platelets to a number of external factors, such as temperature, transportation, etc.As far as morphological examination is concerned it appears that many observers pay too little attention to morphological abnormalities of the erythrocytes.The results with blood group serology point to some difficulties with Rh-factor determination.