Brugia malayi microfilarial excretory-secretory (mf ES) antigens obtained byin vitro maintenance of mf are important tools in the immunodiagnosis of bancroftian filariasis. To increase the yield of mf ES products, the effect of nutritional supplements on the culture medium (RPMI 1640) and the maintenance temperature were studied. Supplementation of RPMI 1640 medium with organic acids and sugars of Grace's insect culture medium forin vitro maintenance of 10 lakhs of mf in 40 ml medium increased the yield of mf ES antigen from 152 μg to 364 μg of mean protein, content and the mean antigen titre from 200 to 400. Supplementation with phenyl methyl sulphonyl fluoride (PMSF) and shift in the culture temperature resulted in a further increase in the yield of mf ES antigen to 502 μg of mean total protein with an antigen titre of 800. The modification resulted in a net increase of 3 fold in the protein content and 4 fold in the antigen titre of ES products. The above modifications in thein vitro maintenance of mf did not affect the diagnostic quality of mf ES antigen, which gave a sensitivity and specificity of 80% and 90% respectively in detection of filarial IgG antibodies inWuchereria bancrofti infected cases.