Previous studies have produced conflicting interpretations regarding the aggregation state of BPTI in solution. Here, pulsed-field gradient NMR self-association measurements have been performed with BPTI under a variety of temperature, pH, salt, urea conditions, and protein concentrations. Relative to the standard proteins, lysozyme, ribonuclease, and ubiquitin, diffusion constants indicate that BPTI dimerizes at concentrations above about 3 mg/mL and below 280 K. At higher temperatures, a marked self- association is observed above 10 mg/mL. The apparent lack of significant effects from variations in pH and NaCl concentration suggests minimal contribution to the aggregation process froth charge-charge interactions. In contrast, in nondenaturing concentrations of urea (2 M), BPTI behaves as a monomer, suggesting that hydrophobic and polar residues modulate BPTI association. The BPTI surface shows that while one side is highly charged, the opposite side, composed mostly of hydrophobic and some hydrophilic residues, is feasible as an interface for BPTI self-association.