There was no compensatory increase in the number of PFCs in other lymphoid organs of carbon treated animals. Similarly, carbon inoculation had no detectable effect on the number of `background' PFCs in the spleens of unstimulated mice.

The time of injection of carbon in relation to time of immunization influenced the effect since injection of carbon 24–48 hours prior to RBC injection resulted in maximum immunosuppression. Injection of carbon 4–5 days before red blood cells resulted in only partial immunosuppression. Treatment with carbon 1–2 weeks prior to immunization had no detectable effect. Similarly, injection of carbon 1 or 2 days after immunization had little or no effect on the peak plaque response.

The decrease in the amount of serum antibody to sheep red blood cells in carbon treated mice was not as marked as that which occurred on the individual cell level. However, most of the antibody in the sera of carbon treated animals was susceptible to 2-mercaptoethanol, even 1 or 2 weeks after immunization. On the other hand, serum antibody from control mice was mainly 2-mercaptoethanol sensitive only during the first week after immunization.

Immunosuppression seemed to be related to a direct effect of carbon since the supernatant fluid obtained from carbon suspensions was not suppressive. Also, washed or dialysed carbon preparations were just as effective as the original preparation in suppressing antibody responses.