In vitro adherence of Pseudomonas fluorescens organisms to the human cornea is correlated with bacterial pili. The role of pili in the attachment to human corneal epithelial cells was studied in an in vitro adherence assay system. A homogeneous, purified pilin preparation showed a molecular weight of 16,500 on SDS-polyacrylamide gels. Within 5 minutes incubation, 5.5 pg of pilin adhere to 15,000 epithelial cells. When epithelial cells were preincubated with purified pilin, a subsequent decrease in adherence of labeled pilin was noted. It appears that pili mediate adherence of Pseudomonas organism to human cornea.