By use of known bounds on constant-weight binary codes, new uppper bounds are obtained on the cardinality of binary codes correcting asymmetric errors. Some constructions are exhibited that come close to these bounds. For single-error-correcting codes some constructions are derived from the Steiner system <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S(5, 6,12)</tex> , and for double-error-correcting codes some constructions are derived from the Nordstrom-Robinson code.