Serum C-reactive protein and neopterin concentrations were measured in samples taken at an early stage in different types of infection to see whether the combination of markers could contribute to the diagnosis of infection and help distinguish between bacterial and viral infections, tuberculosis, and infections due to "other" pathogens. Both markers were significantly raised in all categories of infection compared with controls, and there were significant differences between the means of both markers when comparing several of the categories of infection. Only C-reactive protein concentrations in bacteraemic patients, however, were both sensitive and specific at distinguishing the type of infection. The additional use of neopterin estimation played only a minor part in increasing the specificity of diagnosis in tuberculosis and in viral infections. On the basis of this study it was not considered worth the time and expense of performing neopterin assays in addition to C-reactive protein estimations to differentiate viral from bacterial infection.