A. To determine whether 4 shifts are actually necessary to produce changes in the immune response, mice were shifted once, then allowed to recover for 1 week, 2 weeks or 4 weeks before being injected with 5mg/kg LPS. Control unshifted mice and 4 week-shifted mice are shown for comparison. A single shift produced no significant change in the serum responses of IL-6, IL-18, Macrophage inflammatory protein-2 (MIP-2) or leukemia inhibitory factor (LIF) following this smaller dose of LPS, after any recovery period. The 4-week shifted group exhibited increased levels of all of these factors after challenge relative to controls. B. Peritoneal macrophage quantity did not differ statistically (p > 0.05) among 4-week shifters versus controls (inset), but stimulation of these cultures with LPS induced higher IL-6 release in vitro from cells harvested from shifted mice. HPF: high-powered field (for cell counting).