In vitro experiments with frozen sections of human lung and kidney demonstrated that adhesion of Streptococcus pneumoniae Pn 629 Type 14 and Pseudomonas aeruginosa ATCC 27853 to human cells was mediated by bacterial lectins (adhesins) with N-acetyl-D-glucosamine/D-galactose or N-acetyl-neuraminic acid specificity. Blocking of the lectin binding sites on bacterial surfaces with competitive carbohydrates completely prevented the bacterial adherence, whereas non-specific carbohydrates (D-mannose, D-xylose) did not inhibit adherence.