Self-assembly in aqueous solution of two oppositely charged globular proteins, hen egg white lysozyme (LYS) and bovine calcium-depleted alpha-lactalbumin (apo alpha-LA), was investigated at pH 7.5. The aggregation rate of equimolar mixtures of the two proteins was determined using static and dynamic light scattering as a function of the ionic strength (15-70 mM) and protein concentration (0.28-2.8 g/L) at 25 and 45 degrees C. The morphology of formed supramolecular structures was observed by confocal laser scanning microscopy. When the two proteins are mixed, small aggregates were formed rapidly that subsequently grew by collision and fusion. The aggregation process led on larger length scales to irregularly shaped flocs at 25 degrees C, but to monodisperse homogeneous spheres at 45 degrees C. Both the initial rate of aggregation and the fraction of proteins that associated decreased strongly with decreasing protein concentration or increasing ionic strength but was independent of the temperature.