Using a mouse Thy-1 allelic system (an AKR strain of Thy-1.1 was used as the host and a BALB/c strain of Thy-1.2 as the graft), characteristics of transplant-to-host axon projection and synapse formation of the olfactory bulb (OB) were studied in the hippocampal formation by immunohistochemical and ultrastructural examinations. Thy-1.2-positive axon projections were most prominent in the dentate molecular layer (DM), although there was a labeled-axon-poor layer in the innermost part of the DM. Some of the axons in the DM entered the hippocampus proper, forming two clear borders between the two axon-rich outer layers and the inner axon-poor central layer. These results suggested that axons from the transplanted OB extended into the hippocampal formation, showing axon preferences. In the ultramicroscopic observation of the DM, three different synaptic patterns could be distinguished among the transplant-to-host synapses. These three synaptic patterns are similar to those of the synapses formed by perforant path fiber terminals in the DM, indicating that the transplanted OB forms fiber connections with the dendrites of the dentate granule neurons similar to those of normal afferents.