A polyacrylamide support as defined in claim 3, wherein the polyacrylamide support with attached oligonucleotides has the formula ##STR43## wherein M is --NH-- or oxygen;b is 0 or 2 to 6 (when M is --NH--) and b is 0 to 6 (when M is --O--); c is 1; W is hydrogen or --NHC(???O)--R' where R' is an alkyl group having 1 to 6 carbon atoms; and Z is oxygen. 5.