The frequency of eosinophilia, degranulated eosinophils, and raised serum levels of eosinophil granule basic proteins was determined in ten Nigerians with EMF and fifteen normal controls matched for age and sex, and from the same environment and social background as the patients. Sera from the patients and control subjects were also examined for auto-antibodies, immune complexes, filaria antibodies and immunoglobulins. A mild eosinophilia was observed in four of the patients. The mean direct eosinophil count in the patients (0.407 +/- 0.14 X 10(9)/l) was, however, not significantly different from that of the control subjects (0.399 +/- 0.07 x 10(9)/l). Degranulated eosinophils were absent in the blood and bone marrow aspirates of the patients, and peripheral blood of the controls. It was also found that the mean concentration of eosinophil granule basic proteins (ECP/EPX) in the patients (0.278 +/- 0.1 micrograms/ml) was not significantly different from the mean value for the control subjects (0.371 +/- 0.1 micrograms/ml, P greater than 0.5). Serological studies using Brugia pahangi antigens failed to demonstrate filaria antibodies in the patients and the control subjects. Microfilaria were also absent in their peripheral blood films. Using the indirect immunofluorescent technique, anti-heart antibodies were not demonstrated in the sera of the EMF patients and control subjects. Five (50%) of the patients and four (30.7%) of the normal controls had immune complexes levels far in excess of the upper limit of normal. However, the mean concentration of immune complexes in the patients (16.35%) was not significantly different from the value for control subjects (12.13%, P greater than 0.5).(ABSTRACT TRUNCATED AT 250 WORDS)