Suppression of CFU-s activity by rabbit-anti-mouse brain serum can be overcome by treatment with papain.

The suppression of CFU-s activity which is observed after incubation with rabbit-anti-mouse brain sera (RAMB) can be reversed by incubating the antibody coated cells with papain. Papain treatment does not alter the CFU-s membrane since pre-treatment with the enzyme does not protect CFU-s from the lethal effect of RAMB. This technique should allow the use of fluoresceinated or otherwise labelled RAMB for the selection of CFU-s.