We report the isolation and purification of the nephritis strain-associated protein (NSAP) first described by Villareal et al. (8). Amino acid analysis, and determination of the first 21 amino-terminal amino acids indicated that this 46 kD protein is a streptokinase. Biochemical analysis confirmed that NSAP could act as a plasminogen activator; immunological investigations indicated that NSAP is antigenically different from streptokinase from group C streptococcus, and possibly represents a unique streptokinase. It is this uniqueness that may contribute to the role of NSAP in the pathogenesis of acute poststreptococcal glomerulonephritis.