Female mice were injected with a suspension of 0, 2 ml of 10 per cent v/v sheep red blood cells
(SRBC) and sacrificed on the 5th day for recording of spleen mass and assay of plaque-forming cells
(PFCs) in the spleens. All PFCs from a pool of five spleens and the individual spleen masses were subjected to rigid statistical analysis to verify whether differences from the control groups were significant (P < 0, 05). Three doses of 4 mg cortisol administered at 24-hourly intervals were given at various times relative to the time of immunization to determine the optimum stage at which to inhibit the formation of PFCs. The greatest inhibitory effect was obtained when the first dose was given at -6 h, with a lesser though still significant depression of PFCs with the -30 h dose regimen. Earlier and later injections had relatively little effect.
An attempt was made to determine the response to different dosage levels by injecting doubling doses of steroid starting at the -6 h period. A maximal effect was attained using either 1 to 2 mg cortisol suspension or 0, 5 mg of the soluble pharmaceutical preparations, Efcortolan* and Betsolan*. Inhibition of PFCs appeared to be a more sensitive indicator of steroid action than loss of mass. In addition, spleen mass was determined after injection of either 4 mg cortisol three times or
SRBC. During steroid treatment the spleen mass decreased, but the masses started to increase again almost immediately this treatment was discontinued; the maximum antigenic response, as evinced by attainment of maximal mass, occurred at + 4 days, when the PFC production reached its peak.