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Histamine is an impressive modulator of immune functions at least via its effects on lymphoid cells. Its in vivo effects will not be used practically as long as they produce the profound cardiovascular and pulmonary effects for which the drug is known. A series of 13 congener derivatives and conjugates of histamine was constructed and was tested to investigate whether chemical alterations would result in pharmacologic actions on leukocytes that were more potent and effect specific than histamine. The new compounds, which contained spacer groups of varying lengths between ligand and carrier and with various aromatic modifying groups, showed potencies widely different from histamine when tested in natural suppressor cells. Some compounds showed selective effects on natural suppressor cells in that they were inactive on myocardial tissue, whereas other compounds were selectively active on the myocardium. Some compounds augmented the suppressive capacity of natural suppressor cells in mixed leukocyte reactions via H1 receptors. Our scheme might be more widely extrapolated to other low m.w. immune modulators in an attempt to make them lymphocyte specific. The data also encourage the in vivo testing of selected histamine analogues as selective modulators of immunity. Some of these modulators might be experimentally useful in vivo because they may lack actions in other tissues.