The effects of two inhibitors, fluoride (F-) and zinc (Zn2+), were studied on the formation of mineral by matrix vesicles (MV) in an in vitro system. Kinetically, mineral formation by MV incubated in a synthetic cartilage lymph (SCL) is characterized by three phases: a lag period, a period of rapid uptake, and finally a period of slow uptake. Zn2+ at > or = 5 microM completely inhibited MV mineralization; at < or = 1 microM, it had little effect on rate of ion uptake, but delayed conversion of an OCP-like intermediate into hydroxyapatite (OHAp). F- at > or = 10 microM reduced the rate of rapid uptake by MV and caused the OCP-like precursor to convert to OHAp. When synthetic OCP was seeded into SCL, mineralization ensued and OHAp became the dominant phase. With Zn2+ present, OCP-like features persisted longer; with F-, the OCP-like features were lost more rapidly. When ACP was seeded into SCL, OHAp formed; Zn2+ at < or = 1 microM caused OCP-like mineral to form. Our findings indicate that Zn2+ stabilizes a noncrystalline precursor in MV regulating the length of the lag period; Zn2+ also favors the formation of an OCP-like intermediate whose growth accounts for the rapid uptake phase. This OCP-like phase appears to nucleate formation of OHAp by MV.