We have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven Deep Sea Drilling Project and Ocean Drilling Program sites. The magnetic susceptibility (K) of these samples does not increase linearly with the degree of serpentinization (S). Instead, K remains modest in partially serpentinized samples (S 75%) peridotites in the oceanic crust. Moderately serpentinized peridotites (S < 75%) have NRM values <5 A/m and K < 0.05 SI. Low-temperature oxidation of magnetite is found to lower the NRM and K values of seafloor samples and of cataclastically deformed drilled samples.