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An HEK293S cell line resistant to ricin was prepared by mutagenesis by using ethyl methanesulfonate. It was shown to lack N-acetylglucosaminyltransferase I (GnTI) activity, and consequently unable to synthesize complex N-glycans. The tetracycline-inducible opsin expression system was assembled into this GnTI ؊ HEK293S cell line. Stable cell lines were isolated that gave tetracycline͞sodium butyrateinducible expression of the WT opsin gene at levels comparable with those observed in the parent tetracycline-inducible HEK293S cell line. Analysis of the N-glycan in rhodopsin expressed by the HEK293S GnTI ؊ stable cell line showed it to be Man5GlcNAc2. In a larger-scale expression experiment (1.1 liter) a WT opsin production level of 6 mg͞liter was obtained. Further, the toxic constitutively active rhodopsin mutant, E113Q͞E134Q͞M257Y, previously shown to require inducible expression, has now been expressed in an HEK293S GNTI ؊ -inducible cell line at levels comparable with those obtained with WT rhodopsin.