A form of cell death was studied electronmicroscopically in the regres
sive stage of 4 pigs melanoma. Basically, four types of cells were fou
nd: macrophages, large granular lymphocytes, melanocytes, and melanoph
ages. Morphological changes in cell death were most frequently observe
d in the cytoplasm of tumour cells, in the form of dilatation of granu
lar endoplasmic reticulum and/or vacuolation of cytoplasm close to the
cell surface together with crater-like cavities. Rarely, condensation
and margination of nuclear chromatin were observed without lesions in
the cytoplasm. The above lesions are typical sings of cells undergoin
g apoptosis.