The use of paramagnetism-based constraints for solution structure refinement is described here for the first time for a high spin iron(III)-containing protein (met-myoglobin). This is a particularly challenging system due to the unfavorably high ratio between its magnetic susceptibility and its magnetic anisotropy, coupled with relatively long electron relaxation times. The cross correlation between internuclear dipole-dipole interactions and the dipole-dipole interaction between nuclei and the time average of the electronic magnetic moment (Curie spin) was measured for 61 backbone NH nuclear pairs and used as structural constraint, as recently demonstrated for systems of lower magnetic susceptibility. (C) 2003 Elsevier Science B.V. All rights reserved.