The goal of this proposal is to create a low cost, lightweight, active noise reducing system to quiet electromagnetic noise interference for magnetocardiography (MCG), especially fetal magnetocardiography (fMCG). The device will operate along one measurement axis, which may suffice for fMCG Although MCG (and MEG) provides clinicians with unique, noninvasive diagnostic measures of biological function, the technology is vulnerable to environmental electromagnetic noise, necessitating the use of massive and expensive magnetically shielded rooms (MSRs), which are impractical for many sites. We propose to replace the MSR for fMCG with a system combining two commercially developed components: active cancellation and reference SQUiDs. Our system will weigh just hundreds of pounds and will be approximately 1110 the cost of an MSR. The system will be a Helmholtz-coil noise reduction device, but will be novel in not being driven by room-temperature (fluxgate) magnetometers, but instead by a set of SQUID reference channels built into the MCG measurement instrument itself. Preliminary results suggest that the noise cancellation will be most effective below 100 Hz, which will cover many signals of clinical interest, and will be especially effective at low frequencies (.01 - 1 Hz) where MSR performance is poorest. The noise reduction system will lower costs and improve performance for research- and clinical MCG instruments, and will help bring noninvasive MCG into the budgetary range of more researchers, clinical researchers, and clinical specialists. This advantage will be especially strong for small scale, lightweight MCG systems, including those that operate with nitrogen temperature SQUiDs. The choice of fMCG application, in comparison with MEG and adult MCG is significant, since noninvasive fetal EKG is impossible; a single MCG channel of data is invaluable (localization is not at issue); and the signal of interest is large (1-10 pT) in relation to MEG evoked fields (50-100 fT). The plan will involve modifying an existing amplifier and coil set, already developed commercially by ETS-Lindgren. Tristan will build a SQUID-based instrument, based on its own commercial systems, from components tested on prior biomagetism applications. The system will be tested in a realistic noise setting, and with known noise sources at varying distances.