AbstractFollowing a dose of 8 Gy ofγ-rays delivered to the entire body of BDF1 mice, the proliferative activity in the crypts of the small intestine changes. The labelling and mitotic activity both fall precipitously, but in the lower regions of the crypt recovery from this fall begins soon after irradiation with cyclic fluctuations. Forty-five to fifty hours after irradiation, control levels are reached after which there is an overshoot. The number of clonogenic cells in the crypt shows a somewhat similar pattern of regeneration and overshoot. It has been assumed that these changes reflect the production of endogenous signals for proliferation and inhibition and these might be extracted by diffusion through the gut wall. We report here that at appropriate times after irradiation stimulatory and inhibitory extracts could be prepared. Appropriatein vivoassay techniques have been developed for testing inhibitors or stimulators making similar use of the patterns of proliferative regeneration after irradiation. Extracts prepared at either 15 h or 39 h after irradiation, i.e. during the phase of active regeneration are quite potently stimulatory on recipient animals 96 h after irradiation (i.e., following the decline from a proliferative overshoot) when injected twice 3 h apart. Extract prepared 72 h after irradiation (shortly after the overshoot peak) is strongly inhibitory when tested on unirradiated animals, or animals 90 h after irradiation, when injected four times 2 h apart. An accompanying paper shows that the stimulatory extract is powerfully active on intestinal cell lines. Thein vitroapproach is currently being used to characterise the stimulatory factor.