The purpose of this study is to determine if YM872 in combination with t-PA can reduce disability and brain damage from stroke. YM872 or placebo will be given as a continuous intravenous (iv) infusion for 24 hours. It is important that the study medication, YM872 or placebo, is administered prior to the completion of the t-PA administration. The clinical effects of YM872 in addition to t-PA will be determined by assessing neurological function and disability scores at follow up visits through Day 90 of the study.