The bone marrow (BM) hosts memory lymphocytes and supports secondary immune responses against blood-borne antigens, but it is unsettled whether primary responses occur there and which cells present the antigen. We used two-photon microscopy in the BM of live mice to study these questions. Naïve CD8(+) T cells crawled rapidly at steady state, but arrested immediately upon sensing antigenic peptides. Following infusion of soluble protein, various cell types were imaged ingesting the antigen, while antigen-specific T cells decelerated, clustered, upregulated CD69 and were observed dividing in situ to yield effector cells. Unlike in the spleen, T cell responses persisted when BM-resident DCs were ablated, but failed when all phagocytic cells were depleted. Potential APCs included monocytes and macrophages, but not B cells. Collectively, our results suggest that the BM supports cross-presentation of blood-borne antigens similar to the spleen; uniquely, alongside DCs other myeloid cells participate in cross-presentation.