Myofibroblasts (Mfs) from rat fat tissues produced a potent endothelial cell growth factor (Mf-ECGF). The growth factor activity found in the conditioned media from primary cultures of Mfs, was labile to heat (80 degrees C for 10 min) and proteinase (trypsin), and did not bind to heparin in the presence of 0.2 M NaCl. Mf-ECGF was partially purified 4760-fold with a recovery of 25% from serum-free conditioned media by sequential carboxymethyl (CM) ion-exchange column chromatography and gel filtration. This Mf-ECGF activity was recovered from the 40 kD region of a non-reducing SDS-PAGE, and from the pH region between 6.5 and 7 of isoelectric focusing, with recoveries of 20% and 65%, respectively. These results indicated that a major portion of ECGF activity in the conditioned media was clearly distinct from other well-known endothelial cell growth factors including fibroblast growth factors (FGFs).