The phase separation of globular protein clusters formed by heat-denatured P-lactoglobulin (beta-1g) in mixtures with the polysaccharide K-carrageenan (K-car) has been studied at pH 7 and 20 degrees C. The effect of the protein cluster size on the phase separation was investigated by preparing clusters with radii between 20 nm and 1 mu m. The formation of protein rich microdomains led to an increase of the turbidity starting at a minimum K-car concentration that decreased with increasing cluster size, but was only weakly dependent on the protein concentration. The size and number of microdomains do not depend much on the cluster size, but their density decreases with increasing cluster size leading to a lower turbidity.