Seeing blood in the ejaculate is alarming but is most often benign, particularly in younger men. Common causes include inflammation or infection of the prostate (prostatitis) or seminal vesicles, recent prostate procedures or biopsy, and sometimes minor blood vessel issues. In many cases no specific cause is found and it clears within a few weeks.
It warrants more careful assessment when it is recurrent or persistent, when it occurs in men over about 40, or when accompanied by other symptoms such as painful urination, blood in the urine, fever, or pain, since occasionally it relates to infection, high blood pressure, or other prostate conditions.
See a doctor so they can check your urine, blood pressure, and prostate, and review any recent procedures. Seek prompt care if you also have fever, severe pain, difficulty urinating, or blood in your urine.