Determination of tumor cell maturity and induction of cell differentiation are significant issues in oncology. We studied here in vitro the effects of cyclic AMP and cyclic GMP on human cervix carcinoma cells (HeLa cells), using normal human cervix cells as controls. Relative plasma membrane fluidity (which corresponds with the adhesive power of the cell, membrane permeability, and the ability to maintain the ionic milieu), cell cycle characteristics, and protein kinase C activity (a regulator of the cell cycle) were determined. In the untreated HeLa cells, membrane fluidity and protein kinase C activity were increased versus the controls. Administration of dbcAMP decreased the membrane fluidity and the protein kinase C activity, prolonged the G0/G1 phase of the cell cycle and shortened the S + G2 + M phase; with dbcGMP, the opposite changes were recorded (all findings, p < 0.05). Findings from HeLa cells treated with dbcAMP approached those from the normal controls. Each of the parameter studied reflected the differentiation of the HeLa cells during dbcAMP treatment. They may be of potential use for the determination of tumor cell maturity in clinical oncology.