Protein fibers (e.g., actin filaments, microtubules, and sickle cell hemoglobin polymers), formed by reversible association of quasispherical protein monomers, contain substantial solvent. At high concentrations, solutions of these fibers become non-ideal due to interactions between the elongated particles. One manifestation of this non-ideality is the spontaneous alignment of the fibers. Model calculations, involving simple intra- and inter aggregate interactions leading to polymerization and alignment, agree well with osmotic pressure measurements of sickle cell hemoglobin solutions in the region of the phase transition. However, at higher concentrations the theory cannot fit the experimental data unless solvent is gradually squeezed out of the fiber under osmotic stress. We find that a linear potential for the fiber dehydration gives osmotic pressure results consistent with the experimental data. Under these conditions, substantial fiber dehydration occurs with increasing protein concentrations beyond the alignment transition. This indicates that particle interactions sufficient to cause alignment are also sufficient to squeeze significant amounts of solvent out of the sickle cell hemoglobin fibers.