Leukosialin (CD43) is a major glycoprotein of T lymphocytes which has an extracellular domain of 45 nm in length that is heavily O-glycosylated. Monoclonal antibodies (mAb) to the extracellular domain of human leukosialin induce aggregation of T lymphocytes, monocytes and some cell lines that express leukosialin. The aggregation was reported in one case to be inducible by Fab fragments. In the present study, nine mAb specific for rat leukosialin were tested as inducers of thymocyte aggregation and all were effective. The level of aggregation was reduced by metabolic and cytoskeletal inhibitors, by removal of divalent cations and by reducing the temperature from 37 degrees C to 4 degrees C. The aggregation produced by mAb specific for certain epitopes was less sensitive to these inhibitors than aggregation induced by mAb to other epitopes. To examine the requirement for cross-linking, Fab fragments of four of the antibodies were tested and found to be inactive except for those derived from the OX75 mAb. However, OX75 Fab showed a tendency to dimerize, and monomeric OX75 Fab obtained directly after gel filtration was unable to induce aggregation. Thus induction of rat thymocyte aggregation by anti-leukosialin antibodies requires bivalent cross-linking and maximal aggregation is dependent on energy and an intact cytoskeleton. Mechanisms of antibody-induced aggregation are considered and it is proposed that in the case of leukosialin the antibodies may cross-link cells to overcome inherent repulsion between them and that subsequently other adhesion molecules complete the clustering process.