ones rernarkably at the c ritical stage. Though the curves of erythrocytes and platelets lay on normal Japanese ones after 2 years, the curve of leukocytes still displaced slightly to the left of normal one even after 6 years. 4. Cytogenetics Follow-up of chromosome observations in blood celis has been performed since 1964. Even 20 years after exposure, cells with chromosome abnormalities (both Cu and Cs cells) exist in the peripheral lymphocytes with much higher frequencies than in general population. The frequency of Cu cells (dicentrics +rings), however, was decreasing. On the other hand, Cs cells remained fairly constant in the frequency of around 2%. The frequencies of the chromosome abnormalities were found to be corresponding to the severity of injuries indicated by minimum leukocytes count at the critical stage. Inthe. bone marrow, cells with chrormosome abnormalities (Cs cells) occurred rather constantly with frequencies of more than 2% in all the 4 times of samplings carried out 13-17 years after exposure. 5. Spermatopoiesis Number of spermatozoa decreased about 2 months after exposure, and azoospermia was found. Both fall of motility and morphological abnormalities: of spermatozoa were also observed. recovery were noticed about 2 years after exposure. Indications of Then most of the patients got healthy children. The testicle of the fatal case, which died 206 days after exposure, showed extremely reduced spermatopoiesis. 6. Other findings Slight disturbances of the liver function were found in a few cases at the time of hospitalization. Later it became more obvious. One of the fishermen, who revealed remarkable hematological disturbances, died from liver damage. During follow-up studies elevated values of ~ 9001988 92 -