Abstract: A method to obtain selected individual polynucleotides or mixtures thereof each of which encodes a peptide and at least one polynucleotide of the mixture encodes a peptide having a target property. The polynucleotides of the invention present in the mixture in detectable, retrievable, and clonable amounts are expressed in a host organism for screening for the target activity. The invention features the ability to synthesize controlled random polynucleotides to produce a predetermined mixture of polynucleotides and to avoid synthesis of a stop codon by adjusting the proportions into which the synthesis pool is subdivided and by adjusting the proportions of activated nucleotides added at each coupling step. A polynucleotide encoding a peptide having a target property can be selected and sequenced to deduce the amino acid sequence of the peptide.