hello,
EITHER is used before a singular noun. Use a singular verb after it:
EXAMPLE: Either explanation is reasonable.
EITHER OF is used before a plural noun or pronoun.
In formal speech and writing, use a singular verb:
EXAMPLE: Has either of them called yet?
In informal speech and writing, you can use a plural verb:
EXAMPLE: Have either of them called yet?
Source: Longman Dictionary Of Contemporary English