In this article, we describe the effect of indoleamines: serotonin (5-HT) and synthetic soluble melanin, on the multiplication of HIV-1 in T4 lymphocytic cell lines. The results show that viral production is increased when infected CEM-11 cells are incubated with 5-HT (10(-7) M and 10(-8) M) for 72 hours, whereas at higher doses (10(-3) M and 10(-4) M), there is an inhibition of viral multiplication. As well, when infected CEM cells were cultured in the presence of 5-HT at 10(-4) M, during 15 days, virus production, syncytia formation and cytolytic effect were drastically inhibited. Melanin also inhibits HIV-1 cytopathic effect on MT-2 cells, without cell toxicity, at concentrations of 0.2-10 micrograms/ml. Syncytium formation and cell lysis were also blocked by melanin at concentrations of 0.1 to 10 micrograms/ml, when uninfected MT-2 cells were mixed with HIV-1 chronically infected CEM-11 cells.