Thyroid cells from 14 normal subjects, two patients with Grave's disease, four patients with follicular adenoma and eight patients with papillary carcinoma were cultured in collagen gel. The colonies of these cells were stereoscopically observed and their morphological characteristics were studied with regard to relation with pathological findings of mother tumor, extra-capsular invasion and metastatic potential. For normal thyroid, Grave's disease and follicular adenoma (except for one case), their own characteristic branching type colonies were found. For papillary carcinoma, both branching type and spheroid type of colonies were observed. The ratio of branching type/spheroid type varied individually in the patients with papillary carcinoma. However, the spheroid type was found to trend to be predominant in patients with extra-capsular invasion and/or lymph node metastasis. This means that the observation of spheroid type colonies in collagen gel is suggestive of risk of extra-capsular invasion or lymph node metastasis. From the obtained results, it seemed possible to diagnose poorly-differentiated cells in vitro by morphologically observing colonies of human thyroid papillary carcinoma cells developing in collagen gel.