Mutants of the cellular slime mold Polysphondylium pallidum have been selected using a cell sorter and a fluorescentlabeled monoclonal antibody, mAb 293. This antibody blocks cell adhesion when applied as Fab, and recognizes a carbohydrate epitope containing L-fucose. This epitope is expressed on the cell surface and is present on >10 membrane glycoproteins of different apparent mol. wts. Twenty mutants were obtained which did not bind mAb 293 when tested at 2 h of starvation. After longer periods of starvation the epitope became detectable in the mutants. In all these mutants aggregation patterns were atypical. Generally streams of cells that were radially orientated around aggregation centers were missing or were much shorter than in wild-type. Genetic analysis demonstrated that aberrant aggregation was linked to the alteration in carbohydrate epitope expression. One mutant was unstable and gave rise to subclones in which almost no antibody binding was observed, even after 24 h of starvation, and only few aggregation centers with no streams or very short ones were formed. These results indicate that the capability of the cells to aggregate is correlated with the exposure on their surfaces of the carbohydrate epitope recognized by mAb 293, whose function in development remains to be established.