Plexins B2 and B3 promote Ca2+/Mg2+-dependent cell aggregation. Cells stably expressing the neuronal cell adhesion molecule L1 known to promote Ca2+-independent cell aggregation were used as positive control. Time-dependent decrease of aggregation index Nt/N0 indicates decreasing total number of particles due to an increasing proportion of cells aggregated. Given are mean values of three independent experiments with error bars = S.E. of mean × 1. Aggregation assays were performed in DMEM (A), HBSS without Ca2+/Mg2+ (B), and in HBSS with 1 mM Ca2+/0,5 mM Mg2+ (C).