Two out of six monoclonals (McAbs) produced against subperiodic Brugia malayi infective larva (L3) antigens impaired B. malayi L3 motility independently of human buffy coat cells. Scanning electron microscopy studies showed damage to L3 surface and loss of regular cuticular annulations. The two McAbs (BML 1a and BM1 8b) did not affect B. malayi microfilaria (mf). They were IFAT-positive with B. malayi adult and L3 antigens; other McAbs which did not affect mf or L3 motility were IFAT-negative. All six McAbs did not promote cellular adherence of normal human buffy coat cells to mf or L3.