While collagen type I is often used as a substrate for cell culturing and as a coating in biomedical implants, as far as we know a simple systematic study comparing the effects of the different presentations of collagen type I on the osteoblastic behavior of cells is missing. In this work, human bone marrow cells (hBMCs) were cultured under osteoblastic-inducing conditions, for 21 days, over a layer of adsorbed collagen (monomeric) and on the surface and inside collagen gels (fibrillar). Comparison was made based on three classical parameters; cell proliferation/viability, alkaline phosphatase (ALP) activity, and production of mineral deposits. The three types of collagen type I substrates allowed the adhesion, proliferation, and the osteoblastic differentiation of cells. However, hBMCs behavior was influenced by the monomeric/fibrillar and 2-/3-dimensional nature of the collagen substrates, namely: monomeric collagen favored cell attachment; cells on 2D substrates presented higher proliferation rates during the exponential phase of growth with formation of spiral-like multilayered structures; cells seeded inside 3D collagen gels formed a regular dense cellular mesh and had a low proliferating rate; cells cultured over or inside fibrillar collagen differentiated faster, with the 3D cultures presenting higher levels of ALP activity; and the extension of mineralization was greater for the cultures done over or inside fibrillar collagen. Thus, cells cultured over collagen gels showed both the ability for cell proliferation and for earlier differentiation, a fact that can be exploited in the biomaterials field.