The flocculating activity of an exopolysaccharide, p-KG03, produced by a marine dinoflagellate Gyrodinium impudicum KG03 was investigated. The p-KG03 was a highly sulfated exopolysaccharide that showed strong antiviral activity against encephalomyocarditis virus (EMCV) and immunostimulating activity by NK cell activation. For the industrial applications of p-KG03, as the bioflocculant agent, p-KG03 showed that more than 90% of the flocculating activity in kaolin suspension occurred at concentrations of 0.5 mg/l with the maximum at 1.0 mg/l. However, flocculation decreased from 2.5 mg/l. The flocculation rate increased linearly with concentration and was higher than that observed in commercial products such as polyacrylamide (approximately 1.0 mg/l) or zooglan (approximately 3.0 mg/l). The p-KG03 was an effective flocculant under acidic conditions (pH 3-6) and over a wide temperature range (4-90 degrees C). The presence of cations did not enhance flocculating activity. The average molecular mass, as determined by gel filtration chromatography, was about 1.87 x 10(3) KDa. Galactose was the main sugar in p-KG03, which also contained uronic acid (2.9%, w/w) and sulfate groups (10.3%, w/w). The infrared spectrum of p-KG03 showed absorption bands of carboxylate groups. Thermogravimetric analysis indicated a degradation temperature (T(d)) of 250 degrees C. Several other properties of p-KG03 such as intrinsic viscosity, the rheological behavior, consistency index (k) and flow behavior index (eta) were also studied.