A simple method for repeated in vivo sperm collection from laboratory mice.

Separating the stud from the female just before ejaculation is feasible, easy to learn, and requires no special equipment. The sperm count of collected ejaculates is lower than natural ejaculations, but higher than previous in vivo sperm collection methods achieved. We recommend this simple sperm collection method in mice, especially when the donor cannot be sacrificed and/or repeated sperm collection from the same animal is required for experimental purposes.