Measuring contact angle of water on dried cell or bacterium monolayers allowed van Oss (1) and others (2) to find a correlation between particle hydrophobicity and ingestion by phagocytic cells. The present study was undertaken to understand what was actually assayed with this method. Monolayers were prepared with different cell types at different densities, and they were dried under atmospheres with varying humidity before being studied with scanning electron microscopy and contact angle techniques. It is concluded that a) contact angles are independent of the cell density and substrate structure when more than 30% of the substrate area is covered with cells. b) Initial cell shape should not influence contact angle. c) Contact angles are markedly dependent on the nature of tested cells. d) Contact angles are substantially influenced by the cell drying procedure. e) A very small fraction of the energies we measured would be sufficient to account for cell-cell interactions. Hence these might play a role in some situations of biological interest.