Pseudomonas syringae pv. tomato DC3000 produces Hrp pili under inducing in vitro conditions. A preparation of partially purified extracellular filaments contains HrpA, flagellin and some minor contaminants. HrpA was separated from the major contaminant, the flagellin, by gel filtration to a fraction containing HrpA as well as its three N-terminally truncated forms. These were further separated by two steps of reversed phase chromatography. HrpA and its degradation products were each shown to reassemble into filament structures after denaturation and renaturation showing that HrpA alone is sufficient for formation of filament structures.