New World primate-derived cell lines were instrumental in identifying the primary factors causing glucocorticoid resistance in these primate species. Their use is expanding because it has been recognized that some of these cell lines exhibit differential sensitivity to retroviral infection. To enhance their utility as cell models, we have further characterized one of these cell lines, squirrel monkey–derived B-lymphoblast (SML) cells, using PowerBlot™. PowerBlot™ is a high-throughput, proteomic screen designed to identify differentially expressed proteins. We compared proteins expressed in SML cells and in a human B-lymphoblast (HL) cell line. We found that, relative to HL cells, SML cells overexpress the calcineurin-activated transcription factor nuclear factor of activated T cells 1 (NFAT-1), which exists in a cyclosporine A (CsA)–sensitive dephosphorylated, constitutively active state. We show that there is increased binding of NFAT-1 to deoxyribonucleic acid and greater activity of an NFAT-sensitive human interleukin-2 (IL-2) promoter–luciferase reporter gene in SML compared with activity in HL cells. The increased NFAT activity does not likely result from calcium-dependent activation of calcineurin because cytosolic calcium levels were not different in SML and HL cells. Rather, SML cells express a truncated form of the catalytic subunit of calcineurin that we propose is responsible for the increased activity of the NFAT pathway. Thus, these novel findings first uncovered by a proteomic screen will enhance the value of these New World primate cell lines as “experiments of nature” to gain insight into mechanisms of NFAT activation.