A quantitative immunocytochemical method is described for measuring intracellular thyroglobulin in human thyrocytes grown in monolayer, based on the imidazole-enhanced 3,3′-diaminobenzidine/peroxidase reaction. The influence of ten different fixatives on the content of thyroglobulin immobilized on nitrocellulose filters and in single cells and the influence of thyrotropin and interleukin-1β (IL-1β) on the amount of intracellular thyroglobulin were evaluated. The most suitable fixatives for single cells were 2% carbodiimide, Lison's 'Gendre fluid' and 2 or 4% paraformaldehyde, whereas Bouin, Carnoy A and B, formalin-calcium and Lillie's formaldehyde-acetic acid-alcohol fixative all resulted in reduction of intracellular thyroglobulin. Two per cent glutaraldehyde caused a considerable reduction (p<0.0001). Nitrocellulose filters were not suitable for evaluation of the fixatives, since the results did not correspond to those obtained with single cells. Thyrotropin (1 U/I) increased intracellular thyroglobulin, whereas addition of interleukin-1β to the culture medium for three days caused a dose-dependent reduction with a plateau level at 2×10^-6 gl^-1 (10⁴ U/I_ of interleukin-1β. It is concluded that changes in intracellular thyroglobulin concentration caused by either thyrotropin or IL-1β can be quantified under experimental circumstances where samples for measurements of thyroglobulin-mRNA or extracellular thyroglobulin are difficult or impossible to obtain.