The stability of the warm haemolysins active against trypsinized red cells was studied during storage at -30°C for three months. No loss of activity was observed Possible in vivo fluctuations were studied by examining samples taken daily over a period of one month. It is concluded that the fluctuations sometimes observed when testing for warm haemolysins are not in vivo fluctuations but may be caused by differences in the trypsin treatment.

Abstract. Red cells were treated with 20 different trypsin preparations. When the red cells were used for detecting Rh‐antibodies, the trypsins gave comparable results provided the solutions had the same trypsin activity as measured by Anson units. When testing for warm haemolysins, the effect on the red cells was dependent on the activity as measured by Anson units of the single trypsin preparation, but there were great differences between trypsin preparations which indicates some kind of qualitative differences between these latter. The findings also suggest that there are differences in specificity between warm haemolysins from different people.

Abstract. The stability of the warm haemolysins active against trypsinized red cells was studied during storage at ‐30°C for three months. No loss of activity was observed Possible in vivo fluctuations were studied by examining samples taken daily over a period of one month. It is concluded that the fluctuations sometimes observed when testing for warm haemolysins are not in vivo fluctuations but may be caused by differences in the trypsin treatment.