To optimally modulate a system as complex as the immune system, one must ultimately control its elements individually. Up to this time, use of polyclonal immune stimulants has necessarily involved modulation of a block of immune functions, frequently including undesired activities as well as the activity of interest. We now report selective modulation of individual elements of the immune system by low molecular weight nucleosides, within the context of a fully functional immune system. Loxoribine (7-allyl-8-oxoguanosine) is a well characterized pleiotropic agonist of the immune system in a variety of species, including mouse and humans. In B-cells it binds to soluble cytoplasmic binding proteins, which upregulate transcription upon translocation to the nucleus. By altering specific portions of the loxoribine molecule, multiple distinct, bioactivity profiles have now been obtained. These include: 1) selective augmentation of antibody responses without effects on B-cell proliferation or NK-cell activity; 2) selective enhancement of NK-cell activity and B-cell proliferation in the absence of antibody responses; and 3) selective enhancement of NK-cell activity and antibody responses without B-cell proliferation. Predominant NK-cell responses with minimal B-cell activity of either type also can be generated. The pattern of cytokine mRNA transcription induced is consistent with the spectrum of cellular activities observed. Thus, it is possible to modulate selective activities of the immune system by relatively minor structural modifications of a broad-spectrum immunomodulator in an unseparated cell system.