Chloroform-acetone extract of the aerial parts of Euphorbia aellenii Rech. f. (Euphorbiaceae) was investigated for its diterpenoidal constituents. This led to the isolation of two new and one known cyclomyrsinol-type diterpenes 1-3. The structures were elucidated on the basis of 1D and 2D (1)H and (13)C NMR techniques, and in vitro immunomodulatory activity was evaluated by standard proliferation of human peripheral blood lymphocytes. Results showed that all the three compounds were found to inhibit lymphocyte proliferation significantly (p < 0.05) at 50 μg/ml concentration. Among them, compound 2 showed more activity against phytohemagglutinin-activated T-cell proliferation with an IC(50) of 40.4 ± 9.35 μg/ml.