The influence of recombinant bovine prolactin (PRL) and recombinant bovine growth hormone (GH) was examined on the popliteal lymph node (PLN) expression of interleukin-2 receptors (IL-2R) in female Snell dwarf mice and normal litter mates after concanavalin A footpad injection. The absolute number of PLN CD4+, CD8+, or B+ cells of dwarf mice was less than that observed for normal litter mates, but when adjusted for the difference in body weight, only the absolute number of B cells was lower in dwarf animals when compared with normal litter mates. The injection of PRL or GH did not alter the observation. The administration of recombinant bovine PRL to normal animals, but not recombinant bovine GH, increased the expression of IL-2R or unstimulated PLN CD4+ and CD8+ subsets. Hormone administration to dwarf animals, however, did not alter the expression of IL-2R on unstimulated PLN T cell subsets. PLN cells from dwarf animals were poorly activated in vivo after injection of concanavalin A and the level of IL-2R expression induced was only 50% of that seen in the PLN of normal animals. The administration of PRL and GH completely corrected the defective induction of IL-2R expression on PLN from dwarf animals after concanavalin A stimulation. These findings strongly suggest that PRL and/or GH play an important role at some stage of the T cell activation process in vivo. Further studies are needed to precisely identify the defect in the dwarf mice.