Bacterial infections are the major determinants of fatality in severe protein-energy malnutrition (PEM). Unfortunately, these infections are difficult to diagnose clinically. C-reactive protein (CRP) levels were determined in 17 infected and 10 non-infected Nigerian children with severe PEM and compared with age/sex-matched apparently healthy controls. The aim was to study the response of this acute phase protein to bacterial infections as well as to assess its value in the diagnosis of infections in severe PEM. C3 complement protein levels were also determined in the same group of subjects. The major organisms isolated in samples from these subjects were S. aureus and the coliforms. Mean CRP level in the non-infected children with severe PEM was 13.8 +/- 6.21 mg/l and rose to 159.83 +/- 124.07 mg/l in the presence of infection. The mean value in healthy non-infected controls was 2.01 +/- 0.96 mg/l. The difference in the mean CRP levels between the infected and non-infected PEM children was statistically significant at p < 0.01. The mean difference between the non-infected and the control subjects was not significant. Using a diagnostic level of 20.00 mg/l of CRP gave a sensitivity of 85.0% and a specificity of 80.0%. This CRP level is a useful index of bacterial infections in severe PEM. C3 complement protein was low in the non-infected malnourished group, but rose significantly in the presence of infection to values similar to that of the healthy controls. C3 protein thus behaves as an acute phase reactant in the presence of infection in severe PEM, and does not appear to be consumed, probably due to a deficiency in the early components of the complement cascade. This suggests a role for C3 measurement in the monitoring of bacterial infections in severe PEM.