Methods and compositions for male or female contraception are provided. The compositions include an effective amount of netrin-1 to reduce or inhibit sperm concentration in semen of males or to inhibit or reduce fusion of male gametes with female gametes in a female subject Still another embodiment provides a method for diagnosing male infertility by determining the amount of netrin-1 in a sample of epididymal fluid or semen from a male subject, comparing the amount of netting-1 in the sample to levels of netrin-1 in samples of epididymal fluid or semen from fertile males, wherein levels of netrin-1 in the sample from the male subject that are higher or lower than levels of netrin-1 in samples from fertile males are indicative of male infertility in the male subject.