As a step toward optically pumped magnetometers (OPMs) with sub-mm spatial resolution, we demonstrate optical-cavity-enhanced measurement of atomic vapor spin polarization in a microfabricated vapor cell [1]. The technique, based on the Pound- Drever-Hall (PDH) method, measures the line shift of a circularly-polarized cavity mode, caused by the spin-dependent circular birefringence of the vapor. In contrast to other OPM probing techniques, the optical observable is the phase of a single polarization, rather than a differential phase shift as in Faraday rotation. The method appears well-suited to improving the effective optical path in micro-fabricated atomic vapor cells, analogous to what has been done with multi-pass geometries in macroscopic cells. The signal enhancement will be proportional to the cavity finesse, F ≈ 18 in our case (for a blue detuning of 2π × 115 GHz ). We describe the application of the OPM to study the ability of magnetotactic bacteria (MTB) to orient in an external magnetic field, which along with their ability to migrate towards regions depleted in oxygen, make MTB potential candidates for cancer treatment [2].