A study was made of variation in weight of the host lymph nodes, spleen, and thymus, after implantation of transplantable tumors in susceptible and See PDF for Structure non-susceptible hosts. The lymph nodes and spleens of non-susceptible hosts increased in weight during the period when the organs were participating in the immunological response, though an increase also took place in susceptible hosts. Variations in protein nitrogen and pentose- and desoxypentosenucleic acid of the draining lymph nodes of non-susceptible mice were also studied. The protein nitrogen content increased with the weight of the nodes. Increase in the PNA/DNA ratio occurred while the lymph node cells were engaged in production of antibody. Increase in the PNA/DNA ratio was interpreted as an increase in PNA per cell, and therefore of the rate of protein synthesis.