In the preparation of parenteral solutions for the therapy and prophylaxis of thromboses and embolisms, because of the poor solubility of t-PA hitherto either the infusion of very large volumes has been necessary, or else a solution with a low volume and a high t-PA concentration has been prepared at the expense of setting up a non-physiologically low pH of 2 to 5.Hence, the present invention relates to a process for the preparation of a solution of high concentration of a protein having plasminogen activator activity, where an increase in stability and solubility is achieved by adding at least two substances from the group of D- and/or L-amino acids, their salts, derivatives or homologs. This invention also relates to a process for the pasteurization of a protein solution having t-PA activity, and to a t-PA-containing solution prepared by the claimed process, and to the use of this solution as a fibrinolytic in human and veterinary medicine.