The first part of the study will look at results of treatment in the first 24 hours to select the best dose of TNK to carry forward into a more detailed comparison with standard tPA treatment. After at least 100-150 pairs of the best dose of TNK and tPA patients have been enrolled, entry into the study will pause, and the outcomes at 3 months after stroke will be compared to see if the results of TNK treatment are sufficiently promising as an improvement over standard treatment to justify expanding the study to find a definitive answer.