Cecropin B (CB) is a very efficient antimicrobial agent against Xylella fastidiosa. CB activity decreased after incubation with xylem fluid from Vitis vinifera cv. Chardonnay. SDS-PAGE demonstrated the existence of protein(s) in xylem fluid of V. vinifera cv. Chardonnay that interact with CB and inhibit CB activity. The antimicrobial activity of CB was dependent on incubation time and colony size of X. fastidiosa. X. fastidiosa colonies in periwinkle wilt medium plus (PW+), and chemically defined media (3G10-R and CHARD2) showed different patterns in colony size and colony number in these media. After I h of incubation in 10 muM CB, only large-sized colonies were observed in the three media. These results suggested that most of the bacterial cells or small aggregates in suspension were killed by CB. Aggregation of X. fastidiosa cells may serve as a mechanism of protection against CB. (C) 2004 Elsevier Ltd. All rights reserved.