After review of the literature, this thesis describes the extent of cellular responses to selective antigens in vivo, and to antigens and phytohaemagglutinin in vitro in immunised and transfer factor-treated rhesus monkeys. Preliminary in vitro and fractionation studies of rhesus and human transfer factor preparations are also presented. In immunised animals an antigen-specific active sensitisation was detected by the skin delayed hypersensitivity (DH), the mixed leucocyte-macrophage migration (LMMI) and the lymphocyte transformation (LT) tests. In animals treated with rhesus dialysable transfer factor, adoptive transfer of sensitisation, with a specificity related to antigen- sensitivity of the donor(s) (i.e. "Donor-specific") was detected only by the LMMI-test. The relationship between the various tests was determined. In immunised monkeys, the antigen-stimulated LMMI- and LT-reactivities associated with each other and with the DH-reactivity. In contrast, in transfer factor-treated animals, these reactivities were apparently dissociated. Thus, in transfer factor recipients, dissociation either- reflected different sensitivity of the tests or indicated preferential lymphokine production. Phytohaemagglutinin-stimulated transformation was depressed in immunised animals, whilst 1t was elevated in transfer factor-treated monkeys probably indicating an "adjuvant-like" activity. Thus the results suggest that transfer factor had specific and non-specific in vivo activities 1n the rhesus monkey model. The in vitro study of human or rhesus monkey dialysable transfer factor in antigen-stimulated transformation tests showed augmenting and suppressive effects with antigen-immune or non-1mmune transfer factor. The degree of augmentation was related to the degree of antigen-sensitivity of the "recipient" (1.e. "Recipient-specific"). Thus, the in vitro activity of rhesus monkey or human transfer factor was non-specific. Preliminary Sephadex G-25 chromatography of human and rhesus monkey dialysable transfer factor preparations revealed 9-11 fractionable peaks with different absorption ratios, suggesting heterogeneity of composition.