A staining agent in tablet form for making visible dental plaque contains 85 to 95% by weight lactose; 1.5 to 2.6% by weight phloxin B; 2.5 to 3.5% by weight FDA Blue No. 1; 0.15 to 0.25% by weight magnesium stearate. The staining agent is produced by direct compression tableting and can also contain 0.3 to 0.75% by weight peppermint or eucalyptus flavoring agents.