Trace elements like copper, zinc, iron and selenium have a significant influence on the function of the immune system. We studied plasma levels of trace elements in 53 patients with acute bacterial and viral infections. In bacterial infections (septicaemia, pneumonia, erysipelas and meningitis) the plasma concentrations of selenium, iron and zinc were decreased. Plasma copper was unchanged in patients with erysipelas, but increased in other types of bacterial infections. Although the patients with viral infections showed similar shifts of the trace elements as were observed in patients with bacterial infections, the changes were not as pronounced. A plasma selenium value below 0.8 mumol/l was found in only 6% of the patients with viral infections in contrast to 63% of the patients with septicaemia or 57% of the patients with pneumonia. Furthermore, in viral infections 60% of the zinc values were below the mean level of 12.8 mumol/l observed in healthy controls as compared with 90% of the values in patients with sepsis or 92% of the values in patients with pneumonia. The onset of change in trace elements occurred within a few days and persisted for several weeks. These changes seem to be non-specific and are independent of the agent causing infection. The different types of infections were followed by changes in most of the plasma proteins which are known to be associated with an inflammatory reaction. The changes in plasma proteins were most pronounced in patients with sepsis and pneumonia. Patients with sepsis having a high degree of inflammation did not show a positive correlation between the severity of the disease--as judged by plasma proteins--and the alterations of trace elements.