Vector Hematoxylin QS (H-3404), a modification of Mayer s hematoxylin developed especially for immunocytochemistry, provides crisp blue nuclear staining. This formula is ready-touse without filtration, requires no separate blueing step, and stains in less than 45 seconds. Vector Hematoxylin QS contains no mercury and provides excellent color contrast with most commonly used peroxidase and alkaline phosphatase substrates. This counterstain is suitable for use with nonaqueous and aqueous mountants.