The technique described in this chapter enables the culturing of thyroid cells without loss of differentiation and medium change. It is potentially useful for the long-term study of drug effects on the thyroid gland. Human thyroid cells obtained during surgery can be maintained in culture for periods of up to 2 mo without losing morphological or functional differentiation (1). In the clinical situation the thyroid may be exposed to long-term drug or radiation treatment that may have adverse effects on the functioning of the thyroid gland. These adverse effects can be assessed in culture by studying morphological and biochemical changes after exposure to test chemicals, cytotoxics, or radiation and extrapolated to the likely toxicity in humans.