The effect of low environmental temperature on the humoral immune response to sheep red blood cells (SRBC) was investigated in relation to energy metabolism. Two experiments were done with 3- to 6-week-old pullets using two climate respiration chambers. During both experiments a standard temperature regimen of constant 25 C was maintained in one chamber, whereas the temperature in the other chamber was constant 10 C (Experiment 1) or fluctuated between 5 to 15 C (Experiment 2). The relative humidity was held at 70% at all temperature regimens. Feed allowance was such that growth rates at the two temperatures within experiment were expected to be similar. Indeed, growth rate and body weight were not significantly (P greater than .05) influenced by temperature in Experiment 1. Feed consumption at 10 C was 32.7% above that at 25 C. At 5 to 15 C feed consumption was 34.1% higher than at 25 C, whereas growth rate was reduced (P less than .05) by 8%. At 0 and 10 days after injection, differences in hemagglutinin anti-SRBC antibody titers (log2) between temperatures within each experiment were not significant. The 2-mercaptoethanol resistant (2-MEr) antibody titers 5 days after injection were increased by 26% (P less than .05) at 10 C compared to 25 C. The fluctuating low temperature reduced 2-MER antibody titers 5 days after injection by 19% (P less than .05). Differences in total antibody titers 5 days after injection between temperatures were not significant, but they tended to be reduced by low temperatures. No significant thermal effects on plaque forming cell counts and relative weights (percent of body weight) of spleen and bursa of Fabricius were found. Fat gain during the period between day of injection (Day 0) and peak antibody titers (Day 5) was significantly increased at the low temperatures. Thermal effects on protein gain were not significant, but more protein was deposited at 25 C than at the other temperature regimens during this period after injection.