The fate of 14C-labelled PSK in the body was investigated. Although only substances with low mol. wt were observed in blood shortly after the administration, with time, substances with high mol. wt appeared, suggesting absorption of PSK in its original form from the digestive tract. 14C-labelled PSK was distributed in bone marrow, salivary gland, brain liver, spleen, pancreas and tumor. Approximately 70% of 14C-labelled PSK was excreted by expiratory air after 24 h and approximately 15-20% in urine after 72 h. Only a small amount of 14C-labelled PSK was transferred into lymph and bile. The present study on the in vivo behavior of PSK provides an important basis for further analysis of its pharmacological actions.