The 38th mitzvah is that the Kohen gadol is commanded to marry a young1 virgin.

The source of this commandment is G‑d's statement,2 "he must marry a virgin."

In explanation of this, our Sages said,3 "Rabbi Akiva would declare a child illegitimate even if a positive commandment was violated." They explained this statement that, for example, "a Kohen gadol had relations with a non-virgin, who is forbidden to him by virtue of a positive commandment." [It is termed a positive commandment rather than a prohibition] because of our principle, "a prohibition which is derived from a positive commandment is considered a positive commandment." It is clear from this discussion that this is considered a positive commandment [and therefore included among the 613].

The Sages also said, "he is commanded regarding a virgin."

The details of this mitzvah are explained in the sixth chapter of Yevamos and in passages in Kesubos and Kiddushin.