['Our laboratory has previously shown that tumors are more easily established and grow larger after laparotomy vs. laparoscopy. The purpose of this study was to better characterize these differences in tumor growth by assessing tumor cell proliferation via the proliferating cell nuclear antigen assay, which has been shown to be a reliable marker of cellular proliferation.', 'Female C3H/He mice (N = 40) were inoculated intradermally in the dorsal skin with 10(6) cultured mouse mammary carcinoma cells <1 hour before interventions. Anesthesia control mice underwent no procedure. Laparotomy group mice had a midline incision from xiphoid to pubis that was closed after 20 minutes. Insufflation group mice underwent CO2 pneumoperitoneum (4-6 mmHg) for 20 minutes. On postoperative Day 6, tumors were excised from one-third of the mice in each group, and from the remaining mice on postoperative Day 12. Sections were made and stained immunohistochemically for proliferating cell nuclear antigen, and the proliferative index of each tumor was determined by taking the average of proliferating cell nuclear antigen-positive cells in five high-power fields (x450), counted in a blinded fashion with the aid of an optical grid.', 'On postoperative Day 6, the mean proliferative index for the laparotomy group was significantly higher than those for both the insufflation (P < 0.04) and the control (P < 0.001) groups. Of note, the proliferative index of the insufflation group was significantly higher than that of the control (P < 0.01) group. Similarly, on postoperative Day 12, the mean proliferative index for the laparotomy group was significantly higher than for both the insufflation (P < 0.05) and the control (P < 0.005) groups. The proliferative index in the insufflation group was also significantly higher than that of the control (P < 0.04) group.'] Is tumor proliferative index higher in mice undergoing laparotomy vs. CO2 pneumoperitoneum?