TIMI I was stopped in February 1985 because of statistically significant differences in coronary reperfusion rates in the treatment groups; rt-PA was found to be the superior thrombolytic agent. Following TIMI Phase I, the manufacturer of rt-PA changed to a large-scale production method for rt-PA, and the new product was found to have thrombolytic activity and specificity in vitro and in experimental animals comparable to the product manufactured by the old method. However, the TIMI investigators concluded that clinical evaluation would be necessary prior to initiation of TIMI Phase II.