The amounts of arsonoazoalbumin-As74 containing varied amounts of label were followed in tissues of tumor-bearing mice after intravenous injection. The uptake and clearance rates of this material were compared with those of uncoupled arsanilate-As74. In terms of arsanilate, the uptake of a label into tumor from azoprotein approached that of the uncoupled material only with the use of extensively labeled protein (30 moles arsanilate/mole protein). Clearance of arsanilate-As74 was virtually complete in about 24 hours, whereas 25–50 per cent of the initial concentrations remained in tumor 8 days following injection of arsonoazoalbumin-As74.

Large, persistent accumulations of arsonoazoalbumin-As74 were found within liver and kidney even with as little as 1–2 moles arsanilate/mole protein. These resemble the retention found after injection of other dye proteins and curb possible clinical applications. The possibility of overcoming this handicap with more precise, minimal labeling exists.

The uptake in kidney and the rate of disappearance of injected arsonoazoalbumin-As74 from blood varied directly with the amount of arsanilate bound to protein (within the range 2–30 moles of arsanilate/mole protein). The correlation between these factors was poor for liver and tumor and negative for muscle.

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Supported by Contract AT (30-1) 2182, U.S. Atomic Energy Commission, and Grant C-3265, National Cancer Institute, National Institutes of Health.

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