This study has defined the effects of cell concentration, culture conditions and mitogen concentration on the uptake of 3H-labelled thymidine by mononuclear cells (SMC) purified from the spleens of ducks. The mitogen stimulation of cultures containing from 5 × 102 to 2 × 106 cells per well indicated that the optimal concentration of duck SMC was 5 × 105 cells per well. Cultures with concentrations of 1 × 106 cells per well or more took up 3H-thymidine without the need for mitogenic stimulation. The response to each mitogen tested - phytohaemagglutinin (PHA), concanavalin A (ConA) and lipopolysaccharide (LPS) - was distinctive, with between duck variation in the time for peak stimulation, the level of stimulation and the mitogen concentration required for maximum stimulation. The response of the SMC of all the ducks was strongest to stimulation with PHA, the response to ConA was variable and the response to LPs was poor. Supplementation of the media with 20 per cent fetal calf serum instead of autologous duck serum was satisfactory.