CEACAM1 is a signal-regulating, homophilic cell adhesion receptor system expressed in epithelia, vessel endothelia, and leukocytes. Here, we demonstrate that CEACAM1 is expressed also in PC12 cells, both as the common transmembrane isoforms, CEACAM1-L and CEACAM1-S, and as a novel, secreted, differentially spliced isoform. CEACAM1 can have both positive and negative effects on cell signaling. In an attempt to explain this dual behavior, we have initiated computational analysis of the signal-regulating effects of CEACAM1. This suggests that CEACAM1 can exert its signal-regulating activities by discriminating between binding of Src kinases and SHP phosphatases, respectively. Major factors that regulate this discrimination are the expression levels and expression ratios of transmembrane CEACAM1-L and CEACAM1-S, the concentration of secreted CEACAM1, and homophilic binding of CEACAM1 presented by neighboring cells.