Many Experimental and human tumor cell lines have been previously described as being dependent upon exogenous methionine for their in vitro proliferation. The rationale of the experiments described herein was to decrease the in vivo growth of malignant tumors by reducing the exogenous methionine available in diets fed to Wistar AG rats bearing the highly metastatic rhabdomyosarcoma, RMS-J1. The methionine content in the diet was reduced either by replacing casein (diet 1) with soybean protein (diet 4), or by lowering the amount of soybean protein in the diet (from 23 g/100 g to 12 g/100g) (diet 5), or by using a crystalline amino acid-defined mixture as the source of protein (diet 7). In the latter diet homocysteine replaced methionine and allowed the survival of the animals. Diet 4 significantly reduced the mean number of lung metastases without affecting the primary tumor growth. Treatment of RMS-J1 bearing rats with diet 5 led to the decrease of pulmonary invasion (78 and 21 median lung metastases, respectively, in control and treated groups). This diminished metastatic dissemination resulted from the reduced methionine consumption: the lowered casein content in diet 3 (10 g/100 g) as compared to diet 1 (23 g) did not alter primary tumor growth or the amplitude of lung invasion. Moreover, the addition of methionine to diet 5 prevented the diminution of the median number of lung metastases. Replacement of methionine with homocysteine in the crystalline amino acid-defined mixture (diet 7) fed to RMS-J1 bearing rats led to a limited retardation of primary tumor growth (less than 10%) and to a significant decrease in pulmonary invasion: the median number of pulmonary metastases was 28 and 9 for control and treated rats respectively.