Eosinophil cationic protein (ECP) was assayed in nasopharyngeal secretion (NPS) and serum from 42 infants, hospitalized with acute lower respiratory infection, in El Salvador and the results analyzed in relation to etiology of the infection. ECP concentrations were high in NPS, at an average 50 times higher than those found in serum. Exceedingly high levels of ECP (> 1000 micrograms/L) were found more frequently in wheezing than in non-wheezing children (30% vs 7%) and, accordingly, were more commonly found in children hospitalized with bronchiolitis than in those with pneumonia. Excessive levels were significantly more common in girls than in boys. Of the 42 cases, 28 were found to be caused by respiratory syncytial virus (RSV) subgroup A, and 3 by RSV-B, by means of detection of RSV antigen in nasopharyngeal cells. ECP serum levels were moderately elevated during the acute phase of the respiratory infection and increased slightly but significantly, in cases with RSV antigen-positive bronchiolitis, but not in those with pneumonia. The ECP levels in NPS from patients in Sweden who, by antigen detection in NPS cells, were diagnosed as either RSV or para-influenza 3 infection or none of these, were similar. These results indicate that elevation of ECP in NPS is associated with acute lower respiratory infection in general, but particularly pronounced in cases of bronchiolitis. Elevation of ECP is not an exclusive consequence of RSV infection, but may occur to an equal extent in infections caused by other agents.(ABSTRACT TRUNCATED AT 250 WORDS)