Hyperimmune heterologous serum produced in sheep against mouse Ehrlich ascites tumour cells was absorbed with normal mouse tissue and fractionated by DEAE column chromatography into IgG1 and IgG2 fractions. In vitro cytotoxicity test showed that sheep anti-Ehrlich ascites tumour IgGl fraction was cytotoxic to (51)Cr labelled tumour cells whereas IgG2 had no cytotoxic effect. Pretreatment of the tumour cells with the non-cytotoxic IgG2 fraction slightly inhibited the cytotoxic action of IgG1 in vitro.When EAT cells were coated with either IgG1 or IgG2 by preincubation in vitro before injecting intraperitoneally into mice, both fractions protected the animals against tumour growth. Injection of IgG2 and IgGl fractions separately, one before and the other after the injection of EAT cells, resulted in partial protection only. The difference encountered between the in vitro and in vivo findings could be attributed to the host defence mechanisms involved in the in vivo test system.