Sera from a total of 78 patients infected with Wuchereria bancrofti or appropriate controls were assayed for anti-phosphocholine antibodies in an enzyme-linked immunosorbent-assay (ELISA), using phosphocholine as an antigen. Anti-PC antibodies (both IgM and IgG) were observed in patients with all clinical forms of filariasis but, unexpectedly, were not significantly different from those of normal controls. Among the filariasis patients, however, individuals with patent microfilaraemia had significantly lower IgM-anti-PC titres (P less than 0.05) than did those of the other clinical groups. Competitive studies showed that 'naturally occurring' anti-PC antibodies interfere with PC antigen detection. Transient increases in the levels of circulating PC antigen were found by 3 days after treatment of microfilaraemic individuals with the filaricidal drug diethylcarbamazine. The subsequent decreases in antigen levels were generally associated with increased IgM- and IgG-anti-PC antibody levels. Such changes likely affect not only the ability to monitor the parasitological status of such individuals, but also the patient's subsequent immune interactions with filarial parasites.