bFGF was extracted from either mouse, rat and human cell lines or mouse, rat bovine and human brain tissue and partially purified by cation exchange chromatography and heparin-affinity chromatography. When the heparin-affinity purified proteins were probed on Western blots with antisera against either a highly conserved internal bFGF sequence or recombinant 18 kDa bFGF, species-specific forms of bFGF were detected. bFGF proteins from rat and mouse sources were of apparent molecular weight 18,000, 21,500 and 22,000 whereas those from human sources were of 18,000, 22,500 and 24,000. Bovine bFGF proteins were similar to the multiple human bFGFs. The 22.5 kDa and 24 kDa proteins from human cells were also recognized by an antibody specific for the N-terminally extended forms of human bFGF, whereas this antibody failed to detect 18 kDa bFGF. We showed that the differences in molecular weight between human and rat bFGFs are consistent with the predicted ATG (methionine) or alternative CTG (leucine) translational start sites in the 5' upstream sequences of bFGF cDNAs. In addition we show that, irrespective of the species of origin, the larger bFGF proteins may be separated from 18 kDa bFGF by Mono S chromatography.