PURPOSE: To determine simply and exactly juvenilization, by culturing lymphocyte with the addition of lectin, adding ethidium bromide to the cultured liquid to allow both to react, and measuring the intensity of emitted fluorescent.

CONSTITUTION: Heparin-added peripheral blood is diluted with a phosphoric acid buffer solution. Lumphocyte is prepared by collecting a mononuclear layer with a specific-gravity centrifugal method, etc., and phytohemagglutinin, peanut agglutinin, etc. are added to said lymphocyte. Culturing is carried out at about 37°C for 72hr by the use of modified RPMI-1640, MEM, etc. as a medium. Obtained lectin-treated lymphocyte is dissolved by using sodium lauryl sulfate (SDB), etc., and ethidium bromide (EB) is added thereto to allow both to react. The intensity of fluorescence I0 of EB plus SDS, the intensity of fluorescence I1 of EB plus untreated lumphocytes, and the intensity of fluorescence of EB plus SDS plus lectin-treated lymphocytes are measured. Then the increasing ratio of DNA is obtained according to the equation of (I2-I0)/(I1-I0) to determine lectin- induced junvenilization. Since this method is based on the measurement of intensity of fluorescence of EB and DNA complex, no errors due to different conductors occur, and no effect of the number of lymphocytes appears within a range from 0.5×106 to 1.5×106 per ml. In addition, the procedure is simple.