Summary—In a previous study, we identified regions on the surface of tumor cells which act as acceptor sites for putrescine (Put) and studied the competition between structural analogs of Put (N,N′‐tetramethyl‐α,ω‐diaminoalkanes) and Put bound to latex microspheres. A chain of four to seven carbons was necessary for inhibition of Put‐latex binding to the cell surface of human glioblastoma (U251) cells. We show here that under the experimental conditions, N,N′‐tetramethyl‐1,4‐butanediamine and N,N′‐tetramethyl‐1,7‐heptanediamine exhibit an antitumor effect. In a first step (1–48 h after treatment), cells exposed to these compounds show large intracellular vacuoles. We failed to detect any acid phosphatase activity in these intracellular structures revealing that they were not lysosomes. Electron microscopy observations argue for the conclusion that these vacuoles are an hypertrophy of the endoplasmic reticulum (ER) and/or of the Golgi vesicles. Our hypothesis is that this typical effect of the analogs reveals that ER could be a physiological target of endogenous polyamines. At a later stage (6 days after treatment), the cells undergo morphological and biochemical changes: thin and long expansions characterize the cells and the GFA protein is overexpressed. Correlated to both these effects, karyotypic modifications are found in chromosomes 3 and 6. These changes evoke a differentiation of the treated cells. The work provides evidence that N‐methylated polyamine analogs taking the place of endogenous putrescine demon