An ink formulation for marking living tissue comprises a dispersion of carbon black pigment particles encapsulated in water soluble shells, mixed with a pharmaceutically accepted carrier, such as water, and a preservative. A dispersion formulation is preferential to a suspension formulation for marking living tissue because the carbon particles of the dispersion formulation are less likely to aggregate and sediment over time. In addition, the absence of large quantities of additives makes a dispersion formulation less reactive to living tissue, further enhancing its appeal as a biological marker.