Proteoglycans in mineralized (0.5 M-EDTA/4 M-guanidinium chloride-extractable) and non-mineralized (4 M-guanidinium chloride-extractable) matrices synthesized by a mouse osteoblastic-cell line MC3T3-E1 were characterized at different phases of mineralization in vitro. Cell cultures were labelled with [35S]sulphate and either [3H]glucosamine or 3H-labelled amino acids. At the mineralization phase a large majority of proteoglycans were extracted with 4 M-guanidinium chloride (G extract), and at least five species of labelled proteoglycans were identified; dermatan sulphate proteoglycans (DSPG), apparent Mr approx. 120,000 and 70,000), heparan sulphate proteoglycans (HSPG, apparent Mr approx. 200,000 and 120,000) and DS chains with very little core protein. DSPGs weakly bound to an octyl-Sepharose CL-4B column and HSPGs bound more tightly, whereas DS chains did not bind to the column. Amounts of labelled proteoglycans extracted with 0.5 M-EDTA/4 M-guanidinium chloride (EDTA extract) were much less than those in G extract. Although the predominant species in the EDTA extract were comparable with the DS or DSPGs in the G extract, none of them bound to octyl-Sepharose CL-4B, indicating their lack of hydrophobicity. At the nonmineralizing phase a large chondroitin sulphate proteoglycan (Mr greater than 600,000) was found in the matrix in addition to the five proteoglycan species similar to those at the mineralization phase. Although DS chains at the early phase were similar in size to those at the mineralization phase, the ratio of 2-acetamido-2-deoxy-3-O-(beta-D-gluco-4-enepyranosyluronic acid)-4-O-sulpho-D-galactose to 2-acetamido-2-deoxy-3-O-(beta-D-gluculo-4-enepyranosyluronic acid)-6-O-sulpho-D-galactose was less than that at the mineralization phase. These results agree with those of previous studies performed in vivo and suggest that alteration in the synthesis of proteoglycans is involved in the mineralization process. They also suggest that at the osteoblastic mineralization front proteoglycans undergo partial degradation and lose their hydrophobicity.