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The method described by Weisner et al.for stabilizing 99Tcm-hexamethylpropylene amine oxime (99Tcm-HMPAO) with cobalt chloride hexahydrate solution is the most promising developed to date. The aim of our work was to study the behaviour of cobalt during the labelling of leukocytes with 99Tcm-HMPAO stabilized in vitrousing this method. Three parallel labellings were carried out using six blood samples taken from polycythaemic patients. The first set of labellings was performed using 99Tcm-HMPAO, the second set using 99Tcm-HMPAO stabilized with cobalt chloride hexahydrate solution ‘spiked’ with 57Co-chloride, and the third set using 57Co-chloride solution alone. Measurements of radioactivity content were made on the leukocyte pellets after washing at five time points post-labelling (t= 0, 30 min, 1 h, 2 h and 4 h). The data show that leukocytes do not retain cobalt during labelling with stabilized 99Tcm-HMPAO. Studies carried out in parallel demonstrated that the presence of cobalt in the cell-labelling medium had no effect on cell viability.

• A variety of benign and malignant skin lesions was stained for the presence of prekeratin using an unlabeled antibody peroxidase-antiperoxidase method and an antibody raised against human prekeratin protein. No prekeratin could be detected in benign or malignant lesions derived from melanocytes, and prekeratin could not be found in atypical fibroxanthoma, Kaposi's sarcoma, angiosarcoma, dermatofibrosarcoma protruberans, and leiomyosarcoma. Paget's cells did not contain prekeratin. Varying staining intensities were observed in keratinocytic atypical hyperplasias. Of particular interest is the observation that many spindle cell lesions that were difficult to identify with 100% assurance from routine stains could be positively identified as squamous cell carcinoma after prekeratin was found in the atypical cells.(Arch Dermatol 1983;119:476-479)

TO THE EDITOR.— The infiltration of mature fat cells among intradermal nevus cells in melanocytic nevi is a relatively common event and correlates with age.1 Nonetheless, the origin of these cells is uncertain. Various hypotheses for this fatty infiltration have included (1) presence of nevus lipomatosus in melanocytic nevi,1 (2) fatty metamorphosis of nevus cells,2,3 and (3) development of fat cells from mesenchymal elements.1 Since nevus lipomatosus is usually a congenital lesion and the age of patients with melanocytic nevi that contain fat is usually greater than 50 year s,1 the first explanation is unlikely. With regard to the second possibility, Maize and Foster2 could find no evidence histologically of transition from nevus cells to lipocytes, making this interpretation less credible.S100 protein is an acidic calcium-binding protein that is found in neuroepithelium. In the skin, this substance is found within melanocytes, Schwann's cells, Langerhans'