IGFs were extracted from Cohn fraction IV of human plasma using ultrafiltration of acidified paste as the initial step. Further purification, including HPLC as the final steps, yielded seven IGF-like peptides: two with acidic pI (A1, A2), two with neutral pI (N1, N2), and three in the basic region (B1, B2 and B3). B1 was identified as IGF-I and N1 as IGF-II. The other peptides were further characterized with respect to their molecular weight and by N-terminal amino-acid sequencing. B2 and B3 are IGF-I-like, Al and A2 and N2 are IGF-II-like. Two of the peptides (A2 and B3) appear to be two-chain forms of IGF-II and IGF-I, respectively, as shown by structural analysis and polyacrylamide gel electrophoresis. One peptide (Al) appears to be a new variant of an IGF-II derivative with a substitution of Ser by Cys in position 29. Further analysis involved reactivity in radioreceptor assays for IGF-I and IGF-II. N2, Al and A2 are IGF-II-like, whereas B2 and B3 are IGF-I-like, though there are important differences with the main IGFs. Similar results were obtained in IGF-I and IGF-II C-peptide radioimmunoassays. The physiological significance of these peptides is unknown. They offer interesting perspectives for structure-function analysis.