This study describes the susceptibility to dengue virus infection of a monocytic cell line at different states of differentiation. Infectious virus titres increased in undifferentiated U937 cells following infection with clinical isolates but only when the cells were infected via their Fc receptors. No. c.p.e. was observed and virus was not secreted into supernatant fluid. Once differentiated, cells were susceptible to infection either with virus alone or with virus-antibody complexes. Infection was cytolytic and virus was released into the supernatant fluid. Similar results were obtained with freshly isolated peripheral blood monocytes. Increased blood vessel permeability, which occurs in dengue haemorrhagic fever and dengue shock syndrome patients, has been correlated with secondary heterotypic infections and has been postulated to arise from antibody-enhanced infection of monocytes. The data presented suggest a possible mechanism whereby infected monocytes undergoing diapedesis through blood vessel walls might differentiate sufficiently during the process to release virus and cytokines at localized sites on blood vessels.