The effects of synthetic thymosin peptides (T alpha 1 and T beta 4) and their antibodies on the fertilizing capacity of human sperm cells were investigated. T alpha 1, but not the T beta 4, significantly (p < 0.001) increased the human sperm penetration rates in sperm penetration assay (SPA). Antibodies to both T alpha 1 and TB4, which predominantly bound to the acrosomal region of human sperm cell in the indirect immunofluorescence technique (IFT), also significantly (p < 0.001) increased (up to 4.7-fold) the human sperm penetration rates in SPA. The T alpha 1 and antibodies to both T alpha 1 and T beta 4 enhanced spontaneous as well as calcium ionophore-induced acrosome reaction and release of acrosin from the human sperm cells. There was no effect of T alpha 1 and antibodies to T alpha 1 and T beta 4 on percent sperm motility, although they significantly affected various motility characteristics such as velocity, amplitude of lateral head displacement (ALH), and beta frequency--the motility parameters involved in hyperactivation phenomenon of sperm cells. Both T alpha 1 and T beta 4 were detected in the seminal plasma of fertile men, and the levels of T alpha 1 were significantly (p = 0.002) lower in the seminal plasma of infertile men having defective sperm function. These results indicate that the thymosin molecules, especially T alpha 1, may have a role in human sperm capacitation leading to acrosome reaction. These findings also suggest that the T alpha 1 may find clinical applications in the specific diagnosis and treatment of male infertility in humans.