Although a large number of physical techniques have been successfully used to investigate many of the properties of poly-L-proline, the work reported here has used a combination of osmometry, light scattering, viscometry, and sedimentation studies to reveal a new aspect of this model biopolymer. Experiments were made both on solutions (propionic acid) of poly-L-proline Form I and Form II and on solutions (propionic acid with a threefold dilution of n-propanol) in which Form II was in the process of converting to Form I. The results indicate that an increase in the measured molecular weight accompanies known optical activity and intrinsic viscosity changes which occur as Form II becomes Form I. It appears that the molecular weight determined at infinite dilution for poly-L-proline I is approximately twice that found for poly-L-proline II, and evidence for concentration-promoted aggregation beyond the level of a dimer has been noted in these Form I solutions. Based on these facts and on the information obtained about the particle shapes, it is proposed that this association occurs by a side-to-side binding of two macro-molecules. Discussion is directed toward how these experimental findings can be incorporated into the established concept of the Form I conformation. Light, scattering experiments were also performed on solutions (propionic acid 3 M in LiBr) in which the high-salt, or collapsed, form of poly-L-proline had been generated from either Form II or Form I material. These results showed that the dissolved particles are composed of single chains and are significantly smaller in size than found in solutions of either form and it appears possible that in the collapsed form poly-L-proline might be represented by rodlike macromolecules possessing trans-peptide bonds and a conformation with a relatively small helical pitch.