The placental syncytiotrophoblast is of paramount importance in optimising feto-maternal interactions. Syncytiotrophoblast is generated by the differentiation and fusion of underlying cytotrophoblasts. This process is aberrant in complicated pregnancies. We hypothesized that cell cycle withdrawal determines the phenotypic decision-making of cytotrophoblasts. We therefore investigated the effects of broad-spectrum mitotic inhibitors on cytotrophoblast differentiation. Villous tissue was dissected from term placentae of normal pregnancies and cultured on Netwell supports. Over 48 h, the original syncytiotrophoblast was detached and underlying cytotrophoblasts exposed. The resulting villi were treated with mitotic blockers (Ara-C, colcemid, cyclohexamide, doxorubicin hydrochloride, hydroxyurea, L-Mimosine, purvalanol A). The media was recovered and analysed for lactacte dehydrogenase (LDH) and human chorionic gondadotrophin (hCG), markers of tissue viability and cytotrophoblast differentiation, respectively. The resulting tissue was processed for proliferative activity thorough Ki-67 immunorecognition. Colcemid, cyclohexamide, hydroxyurea, and purvalanol A showed significant cytotoxicity over 48 h incubation. Villous tissue exposed to 0.01 mM and 0.1 mM Ara-C, doxorubicin hydrochloride and L-Mimosine showed no increase in liberated LDH. hCG production increased exponentially with cytotrophoblast differentiation. Higher concentrations of Ara-C and L-Mimosine significantly encouraged hCG production. In addition, total cell and cytotrophoblast proliferation were reduced with Ara-C and L-Mimosine treatment. The inhibition of DNA synthesis and replication with Ara-C and L-Mimosine suppressed active proliferation of villus components and exaggerated the biochemical differentiation of cytotrophoblasts. Cell cycle disruption is therefore a basic trigger for cytotrophoblast differentiation. This approach provides a mechanism for encouraging syncytiotrophoblast formation and may hold benefits for conditions where syncytiotrophoblast cover is attenuated. © 2006 Elsevier Ltd. All rights reserved.