Oligomeric guanidines are highly efficient biocides against a broad spectrum of microorganisms. However, their antitumor effects have not been studied so far. We investigated an antiproliferative effect of Akacid-medical-formulation (AMF), a member of the oligoguanidine family of biocides, against solid cancer cell lines and primary cells by measuring [3H]-thymidine incorporation. Additionally, we examined cell cycle distribution in two AMF-sensitive prostate cancer cell lines (DU-145, LNCaP) using flow cytometry. Finally, the influence of AMF on cell cycle regulatory molecules and intracellular kinase cascade-related signaling molecules was assessed. We found that AMF has variable antiproliferative effects on all tested cells. In DU-145 and LNCaP cells, flow cytometric studies showed a reduction of S-phase with a maximum extent of 24 and 58%, respectively. This was associated with a decrease in expression of cyclin D1, cyclin-dependent kinases 2 and 4, while having varying effects on expression of cyclin E and p27. Additionally, reduced phosphorylation of Erk1 and Erk2 was found, whereas expression of phospho-Akt1 remained unchanged. Herein we report for the first time that AMF exerts potent antiproliferative activity against various malignant cell lines, including those of prostate. We therefore recommend further investigation of the anticancer activity of this biocidal oliguanidine.