The type of pyrrhotite, the direction and intensity of the natural remanent magnetization, and the anisotropy of the susceptibility have been determined for samples (101) of massive sulfides from the Strathcona, Falconbridge, Little Stobie, and Copper Cliff North Mines. The results are mutually consistent and indicate an appreciable positive anomaly over Copper Cliff North, weak anomalies over Little Stobie and Falconbridge, and very low magnetic relief over Strathcona. The aeromagnetic maps show a 100γ positive anomaly over Copper Cliff North, and a weak positive anomaly over Falconbridge, but no anomaly over the other deposits. The Falconbridge and Little Stobie deposits on the South Range occur at the base of the irruptive and contain a very high proportion of the magnetic Fe7S8, while the Copper Cliff North Mine, which is situated in a N-S trending offset, also contains appreciable quantities of nonmagnetic Fe9S10. The distribution of the Fe7S8 and the shape of the deposits with respect to the local geomagnetic field, suggest that the ore deposits on the South Range of the irruptive are more likely to produce marked anomalies than those on the North Range. However, on a detailed scale, deposits at the base of the irruptive on the North Range (Strathcona Mine) also contain very high proportions of Fe7S8, which could give rise to significant local anomalies, in a detailed survey on the surface, or in the mines. Further testing in other areas is required to establish the usefulness of studies of this type with the objective of finding guides for the selection of magnetic anomalies of interest in exploration.