The mammalian lamin B2 gene codes for two proteins, the somatic lamin B2 and the germ line-specific lamin B3. Lamin B3 lacks the N-terminus and a part of the alpha-helical rod domain present in lamin B2. These domains are substituted by 84 amino acids unique for lamin B3. When ectopically expressed in somatic cells, lamin B3 causes severe deformation of nuclei which adopt a hook-like configuration. Accordingly, it was proposed that lamin B3 provides the germ line cells with a more flexible nuclear periphery that facilitates spermatogenesis-specific nuclear reorganization events. Here we investigated which protein domains of lamin B3 are responsible for nuclear deformation in transfected cells, and how stable is the nuclear periphery of these cells. Expression of wild-type and mutant lamins evidenced that nuclear deformations are due to the shortened rod domain of lamin B3. Cell fractionation experiments revealed that lamin B3 can be solubilized more easily than lamin B2. Fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) analyses of transfected cells showed that lamin B3 has an increased mobility compared to B2. Our results lead to the conclusion that lamin B3 reduces the stability of the nuclear periphery. They are also consistent with the notion that lamin B3 is relevant to specific properties of the nuclear envelope during spermiogenesis.