The level of calmodulin increases in cells expressing HIV-1 envelope glycoprotein. Although a calmodulin increase is bound to alter many cellular metabolic and signaling pathways, the benefits to the virus of these alterations must be indirect. However, the possibility exists that increased cellular calmodulin benefits the virus by directly associating with nonenvelope viral proteins. We have, therefore, investigated whether calmodulin can interact with HIV structural proteins Gag, p17, and p24. Calmodulin binds Gag and p17 but not p24 in 125I-labeled calmodulin overlays of SDS-polyacrylamide gels. Removal of calcium by addition of EGTA eliminates this binding. A computer algorithm for predicting helical regions that should bind calmodulin predicts that there are two calmodulin-binding regions near the N terminus of p17. Intrinsic tryptophan fluorimetry shows that two peptides, each of which includes one of the predicted regions, bind calmodulin: p17(11-25) binds calmodulin with a 2-to-1 stoichiometry and dissociation constant of approximately 10-9 M2, and p17(31-46) also binds calmodulin with a dissociation constant of about 10-9 M. These binding sites are nearly contiguous, forming an extended calmodulin-binding domain p17(11-46). In H-9 cells, Gag and calmodulin colocalize within the resolution of confocal light microscopy.