In previous work, we found significant associations of horse chromosome 15 (ECA15) microsatellite markers HMSO1 and HTG06 with two horse infections, Rhodococcus equi and Lawsonia intracellularis, respectively. Interleukin-1 beta subunit and interleukin-1 receptor antagonist encoding genes (IL1B and IL1RN) could be considered as candidate genes underlying the associations reported. Therefore, we identified single nucleotide polymorphisms (SNPs) within three interleukin-1 beta functionally related genes: IL1B, IL1RN and Casp1 (interleukin-1 beta converting enzyme/caspasel encoding gene). Using appropriate restriction fragment length polymorphism (PCR-RFLP) and/or single strand conformation polymorphism (PCR-SSCP) markers, their associations with the two infections by genotyping foals from the original study were tested. In addition, the physical localization of one of the two closely located genes, IL1RN, was re-assessed by fluorescence in-situ hybridization (FISH). A statistically significant association between an intronic SNP of the Casp1 gene with R. equi infection was found. The IL1RN gene was localized to 15q13-q14 in agreement with its originally reported physical position.