The S35-label of S35-BSA was detected in the liver tissue of rabbits to the extent of 0.02 per cent (10 µg or ≃ 1014 molecules) of the injected material at 140 days after injection.

The rate of loss of antigen at the termination of the experiment was of such an order that significant amounts would be expected to persist for at least several years.

Data are reported which extend the retention data previously reported on S35-labelled hemocyanin. They indicate that amounts of the order of 0.05 per cent (25 µg.) of antigen material persist at 330 days after injection.

All of the radioactivity of material retained in the liver tissue 6 weeks after injection was immunologically related to the original S35-BSA antigen.

Preliminary studies are reported which indicate that the retained antigen is bound to ribonucleic acid.

A new method is described for the isolation of p-azophenylsulfonate bovine serum albumin from tissue extracts by means of a Dowex 2 adsorbent.

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