The influence of dietary sources of nucleotides on host in vivo and in vitro immuno-hematologic responses in BALB/c (NCI) mice was studied. Adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) were measured in popliteal lymph nodes undergoing proliferative response to syngeneic and allogeneic in vivo stimulation. Supplementation of a nucleotide-free (NF) diet with yeast RNA (NFR) or uracil (NFU) significantly enhanced the host PLN immune response as compared with NF and NF supplemented with adenine (NFA) diets. Levels of ADA and PNP enzymes in the PLNs increased with the alloimmune PLN response of host, and immunosuppression was associated with decreased ADA and PNP activities in lymphocytes following antigenic stimulation. The induction of these enzymes during immune response appears to require dietary sources of certain nucleotides. When bone marrow cells from control chow fed animals were cultured with supernatants (sups) from mitogen activated splenocytes of animals on each dietary group, NF sups significantly decreased (P less than 0.05) the BM proliferative response compared with the response observed with NFR sups, and similar to NFA or NFU sups. When stimulated with purified IL-3, NFR BM cells had higher levels of Thy1.2 or Lyt 1 surface markers as compared with other test groups. In the in vivo splenic colony formation-CFUs assay, spleens from NFR- and NFU-fed animals had a significantly higher number of colonies than spleens from NF- or NFA-fed mice. Thus, NF diet decreases both in vivo lymphoproliferation response to alloantigen and hemopoietic growth factor production, rendering the host splenic environment deficient for stem cell growth. These adverse effects are reversed by RNA supplementation of NF diet. These nutritional studies demonstrate a critical and regulatory role for dietary nucleotides in immunohemopoiesis.