Summary1) Direct analyses of cholesterol in blood cells from 38 men yielded average value of 137.7 mg/100 ml and there was no correlation with serum values in these samples. Standard deviation of these cell values was only 6.5 and part of this small value is accounted for by analytical error. Indirect estimates agreed closely but variability was larger. 2) In 2 dietary experiments on 6 men which produced average changes of −63.8 and + 51.8 mg of cholesterol/100 ml of serum, there were no significant changes in cholesterol concentration in the cells.We acknowledge the help of Dr. Joseph T. Anderson in chemical analyses and of Jaakko Kihlberg in the statistical work.