With the aim of analyzing the effects of prolonged thermal stress or food intake reduction on lymphoid cell proliferation and antibody synthesis, New Zealand White (NZW) male rabbits, both immunized and nonimmunized with Mycobacterium tuberculosis, were kept in individual cages for 24 d at controlled climatic conditions. Both immunized and nonimmunized rabbits were divided into two experimental groups and one control group. The thermal-stressed rabbits (TS) were exposed to a room temperature of 33.5 degrees C, and dietary restricted rabbits (DR) were pair-fed on the basis of the average feed intake of the TS groups and maintained at a room temperature of 18.0 degrees C. The control group (Ctr) was maintained at 18.0 degrees C and was given ad libitum access to feed. All rabbits were maintained at relative humidity 62 +/- 5%. Sera and peripheral blood mononuclear cells (PBMC) were isolated from blood samples collected on d 0, 6, 12, and 24. Sera were used for determining total proteins and immunoglobulins (Ig) specific or not to mycobacterial antigens. Antibodies to heat-shock protein (HSP) were also determined. The PBMC were used to measure cell proliferation and in vitro Ig synthesis. Both experiments in vivo and in vitro suggest that thermal stress and dietary restriction severely affect the immune cell functions. In fact, both stress treatments decreased the capacity of PBMC to proliferate and inhibited the differentiation of B lymphocytes in antibody-secreting cells. However, a recovery of immune cell functions was only observed in vivo after 12 d of treatment, suggesting that other defensive mechanisms may come into play in vivo. Sera collected from both TS and DR rabbits after 24 d presented antibodies to HSP70, suggesting that the analysis of anti-HSP antibodies could represent a useful indicator to reveal the effects of different stress effectors regardless of the nature of the stress.