Objectives and Endpoints: The objectives of this protocol are to: 1) test the safety and efficacy of an inhaled tPA regimen in children with PB; and 2) identify potential candidate biomarkers of inhaled tPA drug response. Safety endpoints will consist of the development of new, active bleeding that is systemic and/or pulmonary and/or new hematuria (defined as gross hematuria or 2+ microscopic hematuria). Secondary endpoints of efficacy will also be measured (e.g., frequency of cast production). Urine and blood will also be collected for the development of potential biomarkers of inhaled tPA drug response.