JOHN, or HAYS (1513–1571)  , margrave of Brandenburg-Custrin, was the younger son of Joachim I., elector of Brandenburg, and was born at Tangermunde on the 3rd of August 1513 . In spite of the dispositio Achillea which decreed the indivisibility of the electorate, John inherited the new mark of Brandenburg on his father's death in July 1535 . He had been brought up as a strict Catholic, but soon wavered in his allegiance, and in 1538 ranged himself definitely on the side of the Reformers . About the same time he joined the league of Schmalkalden; but before the war broke out between the league and the emperor Charles V. the promises of the emperor had won him over to the imperial side . After the conclusion of the war, the relations between John and Charles became somewhat strained . The margrave opposed the Interim, issued from Augsburg in May 1548; and he was the leader of the princes who formed a league for the defence of the Lutheran doctrines in February 1550 . The alliance of these princes, however, with Henry II., king of France, does not appear to have commended itself to him and after some differences of opinion with Maurice, elector of Saxony, he returned to the emperor's side . His remaining years were mainly spent in the new mark, which he ruled care-fully and economically . He added to its extent by the purchase of Beeskow and Storkow, and fortified the towns of Custrin and Peitz . He died at Custrin on the 13th of January 1571 . His wife Catherine was a daughter of Henry II., duke of Brunswick, and as he left no sons the new mark passed on his death to his nephew John George, elector of Brandenburg . See Berg, Beitrage zur Geschichte des Markgrafen Johann von Kastrin (Landsberg, 1903) .