Abstract Alteration in the testicular weight and various germ cell populations was studied in male mice treated with different doses (0.05, 0.25, 0.5, 1.0 and 2.0 mg/kg b. wt.) of teniposide (VM-26) at various post-treatment time periods. Treatment of mice with different doses of teniposide did not significantly alter the testicular weights, irrespective of the drug dose used. Flow-cytometric analysis of germ cells of the untreated control mice testes revealed four distinct DNA peaks corresponding to elongated spermatids (HC), round spermatids (1C), spermatogonia and non-germ cells (2C) and primary spermatocytes (4C). The region between 2C and 4C peaks represents cells that are actively synthesizing DNA (S-phase cells). Treatment of mice with different doses of teniposide resulted in a significant depletion in the relative percentage of spermatogonia from day 2 to 35 post-treatment depending on the drug dose. DNA-synthesizing, i.e. S-phase, cells declined significantly at day 1 post-treatment and continued to decline up to day 70 post-treatment for all the drug doses studied, except 2 mg/kg drug dose at day 28 post-treatment. A significant decline in the relative percentage of primary spermatocytes (4C) was observed at day 7 that continued up to day 70 post-treatment depending on the drug dose. Round spermatids (1C) declined significantly at day 21 post-treatment after administration of 0.25–2.0 mg/kg VM-26. The relative percentage of elongated spermatids showed a significant decline at day 28 after 1 and 2 mg/kg drug treatment. These alterations in different germ-cell populations are reflected in the various germ-cell ratios. The 4C:2C ratio showed a significant decline at day 7 and 14 post-treatment after 1 and 2 mg/kg VM-26 treatment, while the 1C:2C ratio declined significantly at day 21 post-treatment in the mice treated with 0.5 and 2.0 mg/kg of VM-26. 4C:S-phase and 1C:4C ratios increased significantly from day 1 to 70 post-treatment, depending on the drug dose. Our study demonstrates that the treatment of mice with low doses of VM-26 exerts cytotoxic effects on various germ-cell populations.