Agglutination of yeast, human group A and guinea pig erythrocytes by multiple clinical isolates of E. coli, K. pneumoniae, and P. aeruginosa was investigated and correlated with hydrophobicity measurements of each bacterial strain. Hydrophobicity of the isolates, as measured by hydrophobic interaction chromatography, was similarly correlated with in vitro adherence of the microorganisms to buccal epithelial cells. Agglutination and adherence studies were done in the presence and absence of 0.046 M D (+) mannose. Results showed a wide variability of these parameters among the three general of bacteria. Although E. coli designated mannose sensitive by agglutination showed significantly greater hydrophobicity and attachment to buccal cells, there was no direct correlation between hydrophobic retention and adherence to epithelial cells (p greater than 0.5). As a group, K pneumoniae strains adhered in higher numbers than other gram negative species, but this was unrelated to the hydrophobicity or the designated mannose sensitive/mannose resistant adhesin status of the strain. P. aeruginosa isolates failed to agglutinate yeast and erythrocytes and also adhered poorly to buccal cells. A relationship between bacterial hydrophobicity and in vitro adherence was not found.