IN the presence of costimulation(1), Ca2+ influx in T cells leads to activation (transcription of interleukin-2; ref 2) via calcineurin(3,4). In the absence of costimulation, Ca2+ influx results in anergy (interleukin-2 transcriptional block(5)) through an unknown mechanism. Specific attenuation of interleukin-2 transcriptional induction occurs in Jurkat T cells following pretreatment with a Ca2+ ionophore. A >90% block of inducible interleukin-2 reporter gene activity was initiated by transfection of a constitutively active mutant of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase or CaM kinase II)(6), but not by constitutive mutants of CaM kinase IV, calcineurin or protein kinase C. The block was complete six hours after kinase transfection and showed specificity for interleukin-2; there was no change in beta-actin transcription or in c-fos transcription induced by phorbol myristyl acetate, and a Rous sarcoma virus promoter was stimulated threefold. Multifunctional CaM kinase also attenuated interleukin-2 activation by calcineurin plus phorbol ester. T-cell receptor signalling activates multifunctional CaM kinase. These findings suggest that two Ca2+/calmodulin-responsive enzymes, multifunctional CaM kinase and calcineurin, could mediate the divergent effects of Ca2+ signals in T-lymphocyte regulation.