The aim of this work was to study the in vitro effects of new pyrido[2,3-d] pyrimidines, especially 4-(N-phenylethylamino)-5,7-diphenylpyrido[2,3-d] pyrimidine (3a) and 4-(N-benzylamino)-5-(4-chlorophenyl)-7-(3-methoxyphenyl) pyrido[2,3-d]-pyrimidine (3b), on the proliferative responses of human peripheral blood mononuclear cells, cytokine secretion, ATP levels and intracellular redox status. Peripheral blood mononuclear cells were isolated using differential centrifugation on a density gradient of Histopaque. These cells were cultured with the mitogen concanavalin A (Con A) and with different concentrations of the compounds 3a and 3b (1–100 µM). Proliferation (MTT assay), IL-2, INFγ and IL-4 (Elisa kits), ATP levels, cytotoxic effect (micronucleus test) and oxidative markers (intracellular glutathione, superoxide dismutase, catalase, hydroperoxide and carbonyl protein contents) were investigated after 48-h incubation. Our results showed that 3a was immunostimulant at low concentrations but immunosuppressive at high concentrations (50, 100 µM) with a modulation of cytokine secretion. 3b was immunostimulant in a dose-dependent manner. 3a induced an increase in intracellular ATP levels at 1–25 µM but a decrease at 50–100 µM. In the presence of 3b, cell ATP contents increased with 3b concentrations. Peripheral blood mononuclear cell redox markers were consistent with the presence of an oxidative stress induced by high concentrations of 3a and 3b, with a significant increase in the micronucleus frequency in lymphocytes, suggesting cytotoxic effects. However, 3a at low concentrations (1–25 µM) tended to activate antioxidant defense in cells. In conclusion, 3a and 3b modulate cell-mediated immunity, cell energy and redox status at low concentrations.