Polymorphism of serum cholinesterase (SCE, acylcholinacylhydrolase, EC 3.1.1.8) for the E1 locus was studied in the groups of the patients affected with schizophrenia, peptic ulcer, hereditary erythrocytopathies, tuberculosis, thyreotoxicosis, essential hypertension and rheumatic disease. Increased frequencies of I phenotypes (E1uE1a genotype) were found among patients with peptic ulcer (12.3%), hereditary erythrocytopathies (23.2%), and UF phenotypes (E1uE1f genotype) were observed among patients with schizophrenia (2.8%) and tuberculosis (5.4%). The increased frequencies of E1a and E1f alleles in these groups of patients were, as compared to the control group, statistically significant. The value of relative risk for peptic ulcer was 3.39 in individuals of the E1uE1a genotype, those being 3.62 for schizophrenia and 6.92 for tuberculosis in individuals of the E1uE1f genotype. The nature of the other associations is discussed.