Rabbits were challenged intratracheally with 108 Candida albicans or Torulopsis glabrata, and their lungs were lavaged 5, 60, and 120 min later. Initial lavage samples showed significant agglutination of yeasts, followed by the development of larger aggregates in association with alveolar macrophages. To investigate this early agglutination reaction, lungs of normal rabbits were lavaged with heparinized saline, and after alveolar macrophages were discarded, the cell-free lavage fluid was centrifuged at 25,000 g to recover a small, whitish, surface-active pellet (F fraction). The supernatant was concentrated 15-fold by vacuum dialysis (P fraction). When Candida species, T. glabrata, and Saccharomyces cerevisiae were incubated with the F fraction, serial colony counts decreased eight- to 20-fold with every yeast species tested except S. cerevisiae and Candida krusei. Decrease in colony counts was associated with yeast agglutination. The F fraction was further separated by ethanolether extraction, and yeast agglutination was seen only in the protein-rich fraction. Further separation of this protein-rich fraction by polyacrylamide gel electrophoresis yielded three bands, one of which, with a molecular size of about 104 daltons, agglutinated C. albicans. IgA in the P fraction also agglutinated C. albicans, although not as dramatically as the F fraction.