Purified MAL I+II is conjugated to colloidal gold. The surface of the colloidal gold particle displays both electrostatic and hydrophobic properties. For conjugation to gold sol, the electrostatic interactions must be reduced so that the hydrophobic interactions can succeed. The pH of the gold sol is adjusted to give the net charge of the protein zero, or slightly negative. This prevents the aggregation of the protein due to electrostatic attraction while maintaining the hydrophobic interactions. This product comes in a stabilized liquid form and is suitable for light and electron microscopy.