The [American] rule is to place a period after each abbreviation…
Abbreviations of the following titles…are proper in any writing: Mr., Mrs., Messrs., Dr. –Walsh Handbook
According to British usage, if the abbreviation ends with the same letter as the word entire, no period is necessary:
If the abbreviation ends with a letter other than the one that ends the whole word, a period is called for:
However, the British use seems to have fallen to the American use.