Activation of polymorphonuclear leucocytes (PMN) was investigated after incubation of adult human PMN and group B streptococci (GBS) type Ia with a type-specific polyclonal antiserum and a modified porcine surfactant (Curosurf). The level of oxidative metabolism of PMN was studied using a micromethod modification of the nitroblue tetrazolium (NBT) reduction test. GBS alone did not stimulate significant oxygen metabolite release from PMN, and incubation of PMN with surfactant alone resulted in decreased NBT reduction. After opsonization of GBS with a specific antibody, PMN were activated and the increased oxygen metabolite release was not suppressed when surfactant was added to the system. We conclude that the encapsulated GBS strain investigated needs opsonization with specific antibody to increase oxidative metabolism of PMN, and that incubation of PMN and opsonized GBS with surfactant does not interfere with NBT reduction.