A number of amino acid deprivations have been tested in our laboratory, for the selective eradication of tumour cells in vitro. Withdrawal of the essential amino acid, L-arginine, produced the greatest differential effect on cell proliferation. Normal cells ceased to proliferate and remained viable in G, of the cell cycle, while tumour cells attempted to proliferate in conditions unfavourable for growth, resulting in rapid cell death. Of the six tumour cell lines studied here, four displayed the latter response, while the other two responded in an apparently similar manner to normal cells. Most tumour cells cannot arrest in G1 and are therefore defective in G1 cell cycle control (particularly at the R-point). Analysis of a normal cell line, and the two tumour cell lines that survived arginine deprivation, revealed that cdk4 was downregulated, and the cells were found to possess functional p53. The other four tumour cell lines had dysfunctional p53 and did not downregulate cdk4 upon arginine withdrawal, or relied upon cdk6 for pRb phosphorylation. Arginine is required for histone synthesis during S phase. Histone synthesis in the absence of arginine was compared in a normal and a tumour cell line. Normal fibroblasts synthesise histones to support previously initiated DNA synthesis for the first 24 h of arginine deprivation until the cells reach the R-point. However, HeLa cells cannot synthesise adequate amounts of histone proteins, despite continued DNA synthesis and this is to their detriment. A novel cancer therapy has been developed which exploits the differential response of normal and tumour cells to arginine deprivation. Extracorporeal dialysis was used to reduce blood arginine levels in normal and tumour-bearing dogs. Arginine was successfully reduced to <10 M within the first 12 h of dialysis and this low level was maintained for up to 5 days, but arginine was not reduced for a long enough period of time to see significant tumour regression.