Proliferation of SCC-15 cells was altered by HPV18, alone or in combination with HPV16, but not HPV16 alone, in vitro. HPV-transfected and control SCC-15 cells were plated in 96-well assay plates with media containing 10% FBS and were allowed to proliferate for three days. (A) HPV18- and HPV16/18 co-transfected SCC-15 cells exhibited increased proliferation (n = 120, p < 0.01) although HPV16-transfectants exhibited no deviation from control or mock-transfected cells. (B) Relative change in proliferation, measured between day 3 and day 1, revealed that proliferation of the control and HPV-16 transfectant was approximately 2-fold, while the proliferation of HPV18 and co-transfectants increased to approximately 3-fold. (C) Two-tailed t-tests and ANOVA were used to verify the statistical significance of HPV18 and co-transfectants and the lack of significance between controls and HPV16-transfected SCC-15 cells.