Summary Normal isolated human thyroid cells were incubated in tissue culture medium containing I-131 for 24 to 48 hours. The nature and distribution of labeled compounds were determined. Labeled MIT, DIT, T3 and T4 were present following protein hydrolysis of the incubated cells. The MIT/ DIT ratios ranged from 1.3 to 6.7 in the tissue cultures incubated with I-131 for 24 hours, while that for tissue cultures incubated for 48 hours ranged from 0.3 to 0.9. These results indicate that isolated normal human thyroid cells are able to synthesize iodotyrosines and iodothyronines in the absence of intact follicles and colloid.