A specific assay for the measurement of thioinosinic acid, in human lymphocytes, has been developed with a sensitivity of 50 ng of thioinosinic acid per 5 x 106 lymphocytes. 2 Thioinosinic acid is precipitated from purified lymphocytes as the lanthanum salt. Acid hydrolysis results in the formation of 6-mercaptopurine which, when converted into its phenyl mercury derivative, can be easily extracted into toluene. Back-extraction of the toluene layer with 0. IN HCI regenerates 6-mercaptopurine which is then assayed fluorometrically. 3 Blood samples were taken from renal transplant recipients 3 h after an oral dose of 50 mg azathioprine. The results from 5 patients gave a range of 54 to 173 ng of thioinosinic acid per 5 x 106 lymphocytes, with a mean of 110 ng. 4 In an in vitro incubation of azathioprine, ImM with fresh human blood, 160 and 180 ng of thioinosinic acid per 5 x 106 lymphocytes was formed after 0.5 h and 5 h respectively.
5The assay is suitable for the study of the kinetics of thioinosinic acid formation in lymphocytes of patients with kidney transplants. It could also prove useful in the study of thioinosinic acid formation in leukaemia patients undergoing 6-mercaptopurine treatment.