About 30-70% of microsomal hydroxylation of aniline, 0-demethylation of 4-NO2-anisole, N-demethylation of aminopyrine, were lost when whole organs were frozen and kept at low temperatures (0 degrees, -20 degrees, -196 degrees C). When 9000 X g or 105000 X g fractions were prepared from fresh liver and subsequently frozen to different temperatures, there was little or no such loss of activity. The kinetics of the decrease in microsomal enzyme activities was followed during storage of frozen or freeze-dried microsomes at various temperatures. N-demethylation of aminopyrine appeared to be the most sensitive marker of microsome denaturation.