Novel ampholytic terpolymer of N-vinylformamide (NVF), vinylamine (VAm) and sodium acrylate (NA) with low cationic proportion was obtained by hydrolyzing copolymer of NVF and NA (PNVFNA). Solution properties of the polymer were investigated by methods of turbidity and viscosity experiment. The effect of sodium dodecyl sulfate (SDS) on solution viscosity was also investigated. The results showed that the turbidity curves were bimodal, and pH 3.0 was determined as the isoelectric point (IEP). At high salt concentration, polyampholyte effect was noticeable at IEP in viscosity experiment. Although the proportion of VAm in polyampholyte was lower, the solution properties were influenced markedly in the presence of SDS. At IEP, the viscosity increased more rapidly as salt concentration increased when 10(-3) mol/L SDS contained in solution. A mechanism of SDS bound to positive sites on the collapsed globule surface was suggested.