Heptaminol AMP amidate (HAA), a nucleotide derivative, was found to elevate the intracellular cyclic AMP (cAMP) level of cultured mice spleen cells in a time- and dose-dependent manner. Theophylline and imidazole, when added to the spleen cell culture simultaneously with HAA, respectively caused a further rise and a fall of the cAMP level increased by HAA alone. When comparatively higher doses of the T cell mitogen concanavalin A (Con A) were used in the culture, Con A-induced cell proliferation was mildly inhibited in the culture of spleen cells pooled from HAA administered mice in comparison to the culture of spleen cells pooled from saline treated mice. On the other hand, when another T cell mitogen phytohemagglutinin P (PHA) was used in different concentrations in the culture, there was a trend of enhanced cell proliferation in the culture of spleen cells pooled from HAA administered mice in comparison to the responses in the culture of spleen cells pooled from saline treated mice. The present results supported the previous findings that HAA-mediated immunopotentiation was closely related with a cAMP level elevating property of HAA, and the compound also enhanced the function of helper T cells.