Chlamydia pneumoniae, an intracellular microbe, causes respiratory infections and may participate in the development of atherosclerosis. It is able to survive and multiply in macrophages. The susceptibility of monocyte-macrophages from healthy individuals to C. pneumoniae infection in vitro was studied. Intracellular growth of C. pneumoniae, as an indicator of susceptibility to infection, was compared to serum levels of C-reactive protein, soluble CD14 (sCD14), human heat shock protein (HSP)-IgG, human HSP-IgA, C. pneumoniae IgG and IgA antibodies. The production of C. pneumoniae in infected macrophages was highly variable, ranging from 0 to 638 chlamydial genomes per human genome. Chlamydia pneumoniae production associated positively with serum C. pneumoniae IgA (titre: > or =10) and hHSP-IgG and negatively with sCD14 concentration. The association between sCD14 concentration, C. pneumoniae IgA and human HSP-IgG antibodies and C. pneumoniae production was statistically significant only among males. Age and gender did not correlate with the production. We hypothesize that persons whose macrophages cannot restrict the growth of C. pneumoniae are more prone to chronic infection by this agent.