Interleukin (IL)-12 is an inducer of differentiation of T helper (Th) cells towards Th1, whereas IL-10 is mainly an anti-inflammatory cytokine inhibiting Th I functions. Single nucleotide polymorphisms (SNPs) in the regulatory sequences of genes are presumed to be associated with the differential production of cytokines. One IL12B 3'untranslated region (UTR) and five of IL10 gene promoter region SNPs were screened in 152 individuals by genotyping. IL-10 and IL-12 secretion of lipopolysaccharide (LPS), purified protein derivative (PPD) and Staphylococcus aureus Cowan strain I (SAC) stimulated peripheral blood mononuclear cells (PBMCs) was determined. The frequencies of the less frequent IL12B + 16974 C, IL10 -2763 A -3575 A, -1082 G, -819 T and -592 A alleles were 27.4, 32.2, 25.9, 14.8, 9.3 and 8.6%, respectively. Individuals CC homozygous at IL12B 3'UTR had significantly higher IL-12 secretion levels from LPS and PPD stimulated PBMCs than AC heterozygotes (P = 0.03 and P = 0.02) or AA homozygotes (P = 0.02 and P = 0.05, respectively). IL10 -2763 and -3575 SNPs did not show any effect on in vitro secretion levels, whereas the association of proximal promoter -1082 SNP with IL-10 production is confirmed. IL10 proximal and distal promoter SNP distribution with estimated haplotype variations has implicated considerable similarities with the Caucasian populations in Turkey. (c) 2005 Elsevier Ltd. All rights reserved.