The initial aggregation kinetics of single-walled carbon nanotubes (SWNTs) were studied using time-resolved dynamic light scattering. Aggregation of SWNTs was evaluated in the presence of natural organic matter [Suwannee River humic acid (SRHA)], polysaccharide (alginate), protein [bovine serum albumin (BSA)], and cell culture medium [Luria-Bertani (LB) broth] with varying solution concentrations of monovalent (NaCl) and divalent (CaCl(2)) salts. Increasing salt concentration and adding divalent calcium ions induced SWNT aggregation by screening electrostatic charge and thereby suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. The presence of biomacromolecules significantly retarded the SWNT aggregation rate. BSA protein molecules were most effective in reducing the rate of aggregation followed by SRHA, LB, and alginate. The slowing of the SWNT aggregation rate in the presence of the biomacromolecules and SRHA can be attributed to steric repulsion originating from the adsorbed macromolecular layer. The remarkably enhanced SWNT stability in the presence of BSA, compared to that with the other biomacromolecules and SRHA, is ascribed to the BSA globular molecular structure that enhances steric repulsion. The results have direct implications for the fate and behavior of SWNTs in aquatic environments and biological media.