In vitro manipulation of tissue culture conditions may provide a valuable tool for ascertaining carrier status in inborn errors of metabolism. Skin fibroblast cultures from five obligate heterozygotes and three potential heterozygotes for Hunter's syndrome displayed abnormal mucopolysaccharide metabolism after prolonged maintenance in culture or routine freezing. The mechanism for this phenomenon appears to be preferential survival of cells carrying the Hunter gene. Tissue culture techniques appear to permit identification of Hunter heterozygotes.