When "denuded spermatozoa" (spermatozoa stripped of the greater part of their acrosomes and resembling in may respects spermatozoa after acrosomal reaction) of the bull are incubated with 0.1 M 2-mercaptoethanol (pH 8), sperm chromatin is degraded extensively by a protease in the sperm head. The morphological pattern of sperm nuclear dispersion upon in vitro incubation is similar to that observed in the newly fertilized egg. Following disintegration of the outer layers of the sperm nucleus, chromatin dispersion commences from the periphery of the posterior half and proceeds to the anterior end and to the core of the head. Less basic N- and C-terminal portions of bull sperm histone molecules are digested quickly. The central, very arginine-rich portions of the molecules degrade gradually, yielding an heterogeneous series of arginine-rich peptides (molecular weight, 400-1500). Evidence suggests that the protease which is responsible for the degradation of sperm chromatin is a small fraction of acrosin. This fraction of acrosin appears to be arranged along the nuclear surface and to become associated with sperm chromatin during structural changes of the nuclear surface. A similar proteolysis of rabbit, hamster and guinea pig sperm chromatin has also been observed. The resulting pattern of dissolution of the sperm nucleus is proposed as a model of some of the steps involved in male pronucleus formation from the sperm head after fertilization. Histones H2a, H2b, H3, and H4 associated with DNA are relatively resistant to acrosin.