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The ability of concanavalin-A (Con-A) to activate lymphocytes to secrete human lymphotoxin (LT) was tested in the presence of agents known to modify the intracellular levels of cyclic nucleotides (cAMP and cGMP). Lymphocytes were treated with these agents at different stages of activation: (1) during the first encounter with mitogens, and (2) after they had been fully activated and were restimulated. Two agents, Dibutyryl cAMP and theophylline, dramatically inhibit the amount of LT secreted during both “primary” and “secondary” activation by Con-A. In contrast, DL-isoproterenol had a weak effect during primary activation, but greatly reduced the level of LT secreted during secondary activation. Agents which affect the intracellular level of cGMP were also tested. Imidazole had no effect on LT secretion by either primary or secondary Con-A activated cells. In contrast, carbamyl choline greatly reduced LT secretion to a level comparable to Dibutyryl cAMP and theophylline. All agents tested protected, to some degree, the sensitive α-L cell against LT-induced destruction in vitro. Agents which affect the levels of cyclic nucleotides affect both the effectiveness of the aggressor cell and the sensitivity of the target cells in vitro.