pCI528 is a 46 kb plasmid, originally isolated from Lactococcus lactis subsp. cremoris UC503, which causes a decrease in bacteriophage adsorption in lactococcal strains. Cell surface characteristics of two pCI528-containing strains, L. lactis subsp. cremoris UC503 and L. lactis subsp. lactis UC505, were compared with those of their isogenic pCI528-cured derivatives, UC563 and MG1363, respectively. Following centrifugation, strains containing pCI528 produced a loose fluffy pellet. The presence of pCI528 also caused a 'hard' colony morphology on agar media. The fluffy pellet effect mediated by pCI528 could be eliminated by washing cells in 0.05 M-NaOH, which also restored full sensitivity to phage in the L. lactis subsp. lactis UC505 background. GLC analysis of cell wall material indicated that pCI528-containing hosts had significantly elevated levels of both galactose and rhamnose. Marked alterations in the cell surface of strains harbouring pCI528 were observed in electron micrographs.