Prostate cancer is the most commonly diagnosed cancer and the second leading cause of cancer related deaths in men in the United States. Ciprofloxacin is a relatively non-toxic antibiotic that can be easily administered orally with large volume of distribution and good tissue penetration. Studies from others and our laboratory have recently reported its anti-tumor activity in a variety of human tumor cells. In our current experiment, we studied the effect of ciprofloxacin on a hormone resistant prostate cancer (HRPC) cell line, PC-3. Our study shows significant in vitro cell growth inhibition of PC-3 cell line (p=0.0001) and also shows that there is a synergistic increase in the antiproliferative effect of etoposide when these cells are pretreated with ciprofloxacin for 24 h, prior to etoposide exposure (p=0.0001). Western blot analysis of the protein extracts from these cells showed down-regulation of Bcl-2, altering the ratio of Bax:Bcl-2 favoring apoptosis. In our study no significant effect was seen on p21WAF1 expression by the combination of ciprofloxacin and etoposide but there was down-regulation of p21WAF1 gene by ciprofloxacin alone. Ciprofloxacin also inhibited NF-kappaB binding to DNA. Further studies in this area are warranted as the roles of p21WAF1, Bax/Bcl-2 and NF-kappaB may be important molecular events in mediating the antiproliferative and apoptosis inducing effect of etoposide in combination with ciprofloxacin in HRPC cells.