Islets from male B10.BR mice (H-2(k)) were isolated by the collagenase technique, handpicked with a Pasteur pipette, and incubated (either intact or after dispersion with Dispase) for 0, 3, 5, 7, 10, or 14 days in tissue culture medium supplemented with either lymphokine supernatants or recombinant murine interferon-γ. Islets and single cells were examined for IA(k) molecules by use of indirect immunofluorescence. Ia-positive islet cells were identified on the surface of islets incubated with 5-10% lymphokine for >4 days or with 10, 100, or 1000 ng/ml interferon for >6 days. Islets incubated in unsupplemented medium were Ia negative. Incubation with 5% lymphokine induced Ia expression on 10-40% dispersed islet cultured for >9 days. Dual immunofluorescent staining for Ia and insulin revealed that Ia-positive cells included both β- and non-β-cells.