A technique is presented for examining the role played by orientation of fiber bundles in the mechanical behavior of single osteonic lamellae. Cylindrical osteon samples are loaded perpendicular to their axis and, when required, the direction of loading can be changed continually. The main results are: (1) In osteon samples whose fiber bundles change direction in successive lamellae through an angle of about 90° (Type I), circular fractures appear in lamellae whose fiber bundles have a marked longitudinal spiral course, while lamellae whose fibers have an almost transversal spiral course are unaffected. (2) In osteon samples whose fiber bundles have a marked longitudinal spiral course in successive lamellae (Type II), fractures spread radially from the central canal toward the periphery of the osteon, until all the lamellae are affected. (3) These findings are independent of the degree of calcification; they go to strengthen the view that the compactness of osteonic bone is strengthened by the presence of lamellae whose fiber bundles have an almost transversal spiral course. (4) The first fractures to appear are running between collagen fibrils, indicating that the interfibrillar substance is considerably less resistant than the fibrils themselves. (5) In osteon samples of Type I, the fractures which appear in lamellae whose fibers have a marked longitudinal spiral course are circular, and this makes it possible to isolate lamellae whose fiber bundles have an almost transversal spiral course.