Anti‐citrate synthase (anti‐CS) immunoglobulin (Ig)M levels in plasma and pericardial fluid (PF) of patients negative or positive for various anti‐bacterial antibodies. Patients who were positive for Mycoplasma pneumoniae antibodies in the PF (and plasma) had significantly higher levels of anti‐citrate synthase IgM in their plasma (a) and an elevated tendency in the PF (b) when compared to M. pneumoniae antibody‐negative patients (P = 0·031 and P = 0·076, respectively). Boxes show interquartile ranges (IQR), whiskers indicate lowest and highest values, horizontal lines represent medians, dots indicate outliers of 1·5 × IQR, while the asterisks represent the significance (n = 32). MP = M. pneumoniae; YE = Yersinia enterocolitica; CP = Chlamydia pneumoniae; HP = Helicobacter pylori; BB = Borelia burgdorferi; plasma = peripheral blood plasma; AB = antibody.