Static and dynamic light-scattering measurements are reported on zinc-insulin at room temperature (21 i 1Β°C) and pH = 6.88 in 0.1M NaCl aqueous solution. The experiments were performed at very low concentration, in the range 0.12 x 10-4 to 0.90 x lO-'g C I I -~. Within experimental error, we find no evidence for a critical micellar concentration in this system. The aggregation phenomenon starts immediately after preparation of the solutions, and takes several days to cEme to stable equilibrium. The concentration dependence of the diffusion coefficients, 0, = &(l -k&, is negative, and kD was observed to decrease as a function of time, while the aggregate size was found to increase. The equivalent concentration coefficient, -2BM,, obtained from static light scattering, showed a similar behavior, and, within experimental error, was found to be numerically equal to kD. From the relation found between the diffusion coefficient at infinite dilution and the molecular weight of the aggregates, log & = -0.240 log a, -5.077, we deduce that the insulin aggregates are compact structures with a characteristic radius of 0.71 A/(dalto~~)l/~, surrounded by a hydration layer of a thickness of 8.0 A. The equilibrium aggregation number is approximately 10.