FK506 prevented microglial generation of neurotoxic conditioned media following Aβ stimulation. Microglia were stimulated 48 h in the absence or presence of surface-bound Aβ1-42 fibrils (48 pmole/mm2) and 0.1 and 1 μM FK506. Conditioned media from bound Aβ wells alone (negative control), microglia only wells (microglia CM), and Aβ-stimulated microglia wells (Aβ-stimulated microglia CM) +/− FK506 were transferred to 7 day in vitro primary mouse cortical neuron cultures for 72 h. In addition, FK506 (0.1 and 1 μM) was added directly to neurons (negative control) and to neurons along with Aβ-stimulated microglia. Neurons were fixed and stained with an anti-MAP2 antibody and surviving cells counted and averaged (±SD). Stimulations were performed in duplicate and repeated three times. Results are the average of three independent experiments (*p < 0.001 from control).