The relationship of specific chromosomal changes from the normal diploid pattern and the expression of malignancy will be investigated using the Giemsa banding technique. Direct short term chromosomal preparations will be made on all malignant or potentially malignant solid tumors evaluated at the Children's Hospital of Los Angeles. Cytogenetic findings will also be used as a diagnostic criterion for malignancy in solid tumors of equivocal malignant nature. Direct chromosomal preparations will be done on bone marrows from newly diagnosed and untreated leukemic patients. By following the changes from the initial chromosomal pattern prior to treatment, an attempt will be made to discover whether late recurrent disease represents the same abnormal clone of cells or a significant change from the original clone. Long term cultures of childhood tumor cells will also be established and the tumorigenicity of these cell lines tested in antithymocyte treated mice.