A worker exposed to last week's massive radiation leak in Tokai, Ibaraki Prefecture, underwent a transfusion of peripheral stem cells Wednesday afternoon to shore up his blood-forming functions, doctors said.

According to doctors at the University of Tokyo Hospital, Hisashi Ouchi, 35, was exhibiting signs of pneumonia and remained in critical condition.

Hospital staff began collecting blood from Ouchi's brother in the morning in preparation for the transfusion. It has become difficult for the patient to produce blood due to a sustained drop in his white blood corpuscle and platelet count, doctors said.