Appeal by defendant from a judgment of the Supreme Court, Queens County (Sherman, J.), rendered July 17, 1980, as amended September 14, 1981, convicting him of criminal possession of a controlled substance in the third degree, upon a jury verdict, and imposing sentence. Judgment, as amended, affirmed. Defendant's guilt was established by overwhelming proof and we decline to review, in the interest of justice, the unpreserved issues concerning a purported agency defense and the necessity for corroboration of alleged accomplice testimony. Further, it was not improper to admit certain background evidence, which was essential to an understanding of the delivery to, and possession by the defendant on October 3, 1977 of the contraband in issue (see People v Ventimiglia, 52 N.Y.2d 350, 359-360; People v Jackson, 39 N.Y.2d 64). Finally, the replacement of a sitting juror with an alternate after it appeared that the sitting juror knew a witness in the case, was within the court's discretion and did not prejudice defendant (CPL 270.35; see Smith v Phillips, 455 U.S. 209; People v West, 92 A.D.2d 620, 621; cf. People v Blyden, 55 N.Y.2d 73, 78; People v Meyer, 78 A.D.2d 662, 664). Titone, J.P., O'Connor, Weinstein and Rubin, JJ., concur.