The human carcinoma cell line RPMI 2650 produces autocrine factors. These are detected by the ability of RPMI 2650 conditioned medium (CM) to stimulate growth in soft agar of RPMI 2650 cells plated at low density The autocrine activity in crude CM can be fractionated by ultrafiltration into a lower molecular weight (MW) fraction (Rl-30), which concentrates molecules in the X,000-30,000 kDa range, and a higher MW fraction (R30) with molecules greater than 30,000 kDa in a more concentrated form Rl-30 is labile to acid, base and heat treatment, whereas R30 is stable to (and sometimes activated by) these treatments. Boiling of R30, however, renders it labile to acid-, base- and trypsin-treatments CM can be separated into a weakly heparin-bindmg fraction (with stability properties similar to Rl-30), and a non-heparin binding fraction (with stability properties similar to R30) RPMI 2650 cells secrete TGF-a- and TGF-B-hke molecules, but the Rl-30 can be distinguished from these TGFs, and from most other known growth factors, by the unusual combination of acid lability and weak affinity for heparin Since the R30/non-heparin binding fraction is rendered labile by boiling or acid treatment, it may represent a bound or conformâtlonally stable form of a growth factor Growth factor production from RPMI 2650 cells was successfully scaled up to a 2 litre scale.