Cyclophilin B (CypB) was first described and its sequence was reported by Spik et al. in 1991 (1). These authors prepared and characterized recombinant CypB demonstrating that it had prolyl isomerase activity, which was inhibited by cyclosporin (1). At that time, no mitogenic activity was observed for CypB. This may be because of the absence of glycosylation, incorrect folding, denaturation, or to the cellular model used in these initial exploratory experiments (3T3). We wish to report that an earlier unpublished study (2) showed that CypB has growth factor activity on CCL-39 cells at around 30 ng/ml, similar to EGF, acidic FGF, and basic FGF.