The capacity of Coxsackie B3 virus to infect insulin-containing beta cells was studied in human pancreatic cell cultures. Antibody to Coxsackie B3 virus was labeled with fluorescein isothiocyanate, and antibody to insulin was labeled with rhodamine. By use of a double-label antibody technique, three populations of cells were identified: uninfected insulin-containing beta cells, which stained only with rhodamine-labeled anti-insulin antibody; Coxsackie-infected (noninsulin-containing) cells, which stained only with fluorescein-labeled anti-Coxsackie antibody; and Coxsackie-infected insulin-containing beta cells, which stained with both antibodies. Radioimmunoassay showed that intracellular immunoreactive insulin decreased rapidly beginning at 24 hours after infection, and the decrease in insulin roughly paralleled the increase in viral titer. It is concluded that, under in vitro conditions, human beta cells are susceptible to Coxsackie B3 virus.