It has recently been shown that Klebsiella pneumoniae strains adhere to human epithelial cells and that adherence is mediated by mannose-inhibitable adhesins which are also receptors for coliphages T3 and T7. We have now found that Klebsiella strain K59, which adheres to human epithelial cells and carries the receptors for coliphages T3 and T7, adheres to human polymorphonuclear leukocytes (PMN) at 4 degrees C. Strains KRTT1 and KRTT2, which are spontaneous mutants unable to adsorb coliphages T3 and T7 and adhere to human epithelial cells, at this temperature did not adhere to PMN. Adherence of K59 cells to PMN at 4 degrees C was inhibited by D-mannose, by UV-inactivated T7 phages, and by pepsin-digested anti-K59 antibodies absorbed with KRTT1 cells. At 37 degrees C the number of PMN with KRTT bacteria associated was fourfold higher than at 4 degrees C. On the contrary, the number of PMN with K59 bacteria associated at this temperature was fourfold lower than at 4 degrees C. Phagocytosis and intracellular killing experiments performed at 37 degrees C showed that KRTT1 and KRTT2 were phagocytized and killed at a higher rate than K59. After blocking of the mannose-inhibitable adhesins and T3-T7 receptors (MIAT) by D-mannose, UV-inactivated bacteriophage T7, or specific antibodies, K59 cells became more sensitive to phagocytosis and intracellular killing at 37 degrees C. K59 cells lysogenic for prophage AP3 were approximately as sensitive to phagocytosis and intracellular killing by human PMN as strains KRTT1 and KRTT2. Unencapsulated Klebsiella strains isolated from clinical specimens were found to carry MIAT most often. Four such strains were found much more resistant to phagocytosis and intracellular killing than their spontaneous mutants resistant to bacteriophages T3 and T7.