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Color Magnitude Diagrams (CMDs) of individual stars in 8 Globular Clusters inM31 down to about 1 mag fainter (V$\sim 26.5$) than the Horizontal Branch havebeen obtained with the Hubble Space Telescope. In particular, we observed G280and G351 with the FOC (f/96+F430W, f/96+F480LP) while the WFPC2 (F555W,F814W)frames for G1,G58,G105,G108,G219+Bo468 were retrieved from the HST archive. Thecluster metallicities -\fe- range from -1.8 to -0.4. Coupled with sufficientlyaccurate (to $\sim\pm0.1$ mag) measures of the mean brightness of the HB--\vhb--, appropriate estimates of reddening for each cluster, and the adoptionof a distance modulus to M31 of \dmo = 24.43, this has allowed us to yield adirect calibration for the mean absolute magnitude of the HB at the instabilitystrip --\mv-- with varying \fe: M_V^{HB} = (0.13 \pm 0.07)\fe + (0.95 \pm0.09), where the associated errors result from the adopted global errors in themeasure and best fitting procedures. The slope of the derived relation is fullyconsistent with that predicted by the standard and canonical models($\sim0.15$) and obtained by various ground-based observations, while it isonly marginally compatible with higher values ($\sim0.30$), also obtained inthe past. The zero-point, which is crucial to absolute age determinations,depends on the adopted distance to M31 and is moreover affected by anadditional error due to the residual uncertainties in the \hst photometriczero-points ($\sim0.05$ mag, at least). If confirmed, such a calibration of the\mv \vs \fe relationship would imply old absolute ages ($> 16$Gyr) for theoldest Galactic globulars and fairly small age spread among those having aconstant magnitude difference between the Main-Sequence Turnoff and the HB.Comment: 22 pages, processed with the AAS LaTEX macros v4.0, plus 6 .ps  figures, uuencoded, gzipp'ed tar file; fig. 1 (cluster image) will soon be  available via anonymous ftp at boas3.bo.astro.it/bap/files/bap96-28-fig01.ps  -- accepted by The Astronomical Journa

The M(v)^HB Verses [FE/H] Calibration. I. HST Color-Magnitude Diagrams of Eight Globular Clusters in M31