The stability of isoenzymes of alkaline phosphatase from liver, bones and small intestine was compared after addition to inactivated serum in the buffer systems: glycine, 2-amino-2-methyl-l-propanol, diethanolamine and 2-amino-2-methyl-l ,3-propandiol at 37 °C. The mentioned isoenzymes were inactivated to different extents in glycine and 2-amino-2-methyl-lpropanol buffers. In diethanolamine and 2-amino-2-methyl-l ,3-propandiol buffers sufficient stability of isoenzymes is obtained so that only these buffers are suitable for activity determinations of alkaline phosphatase at 37 °C.