A comparison was made of succinyladenylate lyase (SAMP lyase), total serum sialic (TSA), and lipid soluble serum sialic acid (LSA) as early markers of malignancy in three experimentally induced rat tumor models. Elevation of SAMP lyase in 3'-methyl-dimethylaminoazobenzene-induced hepatic tumors at 2 weeks corresponded with microscopic detection of preneoplastic lesions with elevation of LSA occurring 2 weeks later. Elevation of breast SAMP lyase concurred with macroscopic presence of dimethylbenzanthracene involved breast tumors with elevation of LSA occurring 12 weeks later. Neither colon SAMP lyase nor LSA increased in rats bearing colon tumors induced by dimethylhydrazine. The determination of TSA was not a reliable indicator of tumor presence for the three types of tumors investigated. Both SAMP lyase and LSA are very good early indicators of hepatic tumor with SAMP lyase an earlier indicator of breast tumor than LSA.