Strains of Flavobacterium psychrophilum were studied for their ability to adhere and cause agglutination of erythrocytes and yeast cells. Strains of the serotype Th showed low or no hemagglutinating (HA) properties toward human, avian, bovine, and rainbow trout erythrocytes, whereas strains of serotype Fd and Fp(T) exhibited distinct HA properties. None of the strains was able to cause agglutination of yeast cells. Greater adherence specificity toward rainbow trout blood cells was seen for the HA-positive strains. Growth at 5 degrees C, compared to that at 15 degrees C, induced an increase in the hemagglutination of some strains. HA activities of F. psychrophilum were inhibited only by sialic acid (N-acetyl-neuraminic acid), heat treatment at 65 degrees C, and proteinase K treatment and not by any of seven other carbohydrates, periodate oxidation, or treatment with trypsin. The supernatant from washed bacterial cells also showed some HA properties. All strains were shown to be highly hydrophobic by the hydrophobic interaction chromatography test, although some contradictions to the results of the salt aggregation test (showing some strains as less hydrophobic) were seen. These results indicate that the aggregation of F. psychrophilum and erythrocytes depend on a lectin present on the surface of HA-positive F. psychrophilum strains and absent on HA-negative strains. This lectin reacts specifically with sialic acid. The adhesion differences observed for F. psychrophilum strains do not appear to correlate with the virulence but still provide insights into the interaction of F. psychrophilum and rainbow trout.