The serum samples of 50 patients with lymphoid neoplasia, and of 112 control subjects from the normal population were analysed for the presence of antitoxoplasma IgG antibodies by the enzyme-linked immunosorbent assay technique. There was no significant difference between the prevalence rate of antitoxoplasma IgG antibodies in the control subjects (69%) and the patients with lymphoid neoplasia (66%). The prevalence of antitoxoplasma IgG antibody levels of 75 international units (i.u.) per ml and above was significantly higher in the patients (48%) than in the control subjects (18%). Antitoxoplasma IgG antibody levels of 150 i.u. and above per ml indicating active toxoplasmosis were present in 13 (26%) out of the 50 patients with lymphoid neoplasia. None of the sera from the control subjects had antitoxoplasma IgG antibody level up to 150 i.u. per ml of serum.