SummaryPortal occlusion selectively inhibits thein vivoincorporation of [14C]formate into the purines of deoxyribonucleic acid isolated from rat lymph nodes and thymus compared with incorporation into deoxyribonucleic acid thymine. Incorporation of formate into deoxyribonucleic acid purines is also low in perfused submandibular lymph nodes and in lymph nodes incubatedin vitro. A similar depression of formate incorporation into lymphoid tissue deoxyribonucleic acid purines is obtained by treatment with the immuno‐ suppressive drugs azathioprine and 9‐butylazathioprine.