This field study was carried during hot summer to ascertain whether lymphocyte functions of heat-stressed cows undergo changes at the approach of calving, and are modified by oral administration of chromium (Cr). Twenty pluriparous and late pregnant Holstein cows were utilized in the study. During the last four weeks of gestation, seven cows were fed the basal diet (C); seven cows were fed the basal diet plus 8 mg/day of Cr tripicolinate (CrT8); and 6 cows were fed the basal diet plus 16 mg/day of Cr tripicolinate (CrT16). Values of air temperature and relative humidity were recorded continuously at one hour intervals, by use of two electronic data recorders. Rectal temperatures (RT) and respiratory rates (RR) of cows were recorded weekly. On days 10 and one before the expected calving, individual blood samples were taken by jugular venipuncture. Data of air temperatures and relative humidity were utilized to calculate the temperature humidity index (THI). Blood samples were processed for isolation of peripheral blood mononuclear cells (PBMCs), which were then cultured to assess the DNA synthesis after stimulation with mitogens. Mitogens utilized were phytohemagglutinin (PHA, 2.5 µ g/ml), concanavalin A (ConA, 2.5 µ g/ml), and pokeweed mitogen (PWM, 1 µ g/ml). DNA synthesis of PBMCs was assessed by use of an ELISA assay based on the measurements of 5-bromo-2-deoxyuridine incorporation. During the study, mean daily values of THI were 76.1 (SD 2.6) during the day-time (9-19 h), and 65.8 (SD 4.1) during the night-time (19-9). Neither the distance from calving, nor the administration of Cr affected significantly RT or RR of cows. Mean values of RT and RR in the 20 cows were 39.5 °C (SEM 0.4), and 58 (SEM 19) breaths/min respectively. The distance from calving did not affect significantly the DNA synthesis of PBMCs in any of the three experimental groups. Conversely, the DNA synthesis of PBMCs isolated from CrT16 cows and stimulated with PHA was higher (P < 0.05) than that recorded in PBMCs of C cows both on days 10 and three before the expected calving. Results reported herein indicate that heat stress would mask the impairment of the immune functions reported for cows approaching to calving, and that administration of Cr to heat-stressed cows would not be responsible for remarkable amelioration of immune cell functions.