We present estimates for the mean bias of the TOPEX/POSEIDON NASA altimeter (ALT) and the Centre National d'Etudes Spatiales altimeter (SSALT) using in situ data gathered at Platform Harvest during the first 36 cycles of the mission. Data for 21 overflights of the ALT and six overflights of the SSALT have been analyzed. The analysis includes an independent assessment of in situ measurements of sea level, the radial component of the orbit, wet tropospheric path delay, and ionospheric path delay. (The sign convention used in this paper is such that, to correct the geophysical data record values for sea level, add the bias algebraically. Unless otherwise stated, the uncertainty in a given parameter is depicted by ¿&sgr;x, where &sgr;x is the sample standard deviation of x about the mean.) Tide gauges at Harvest provide estimates of sea level with an uncertainty of ¿1.5 cm. The uncertainty in the radial component of the orbit is estimated to be ¿1.3 cm. In situ measurements of tropospheric path delay at Harvest compare to within ¿1.3 cm of the TOPLEX/POSEIDON microwave radiometer, and in situ measurements of the ionospheric path delay compare to within -0.4¿0.7 cm of the dual-frequency ALT and 1.1¿0.6 cm of Doppler orbitography and radiopositioning integrated by satellite. We obtain mean bias estimates of -14.5¿2.9 cm for the ALT and +0.9¿3.1 cm for the SSALT (where the uncertainties are based on the standard deviation of the estimated mean (&sgr;x¿/y), which is derived from sample statistics and estimates for errors that cannot be observed). These results are consistent with independent estimates for the relative bias between the two altimeters. A linear regression applied to the complete set of data shows that there is a discernable secular trend in the time series for the ALT bias estimates. A preliminary analysis of data obtained through cycle 48 suggests that the apparent secular drift may be the result of a poorly sampled annual signal. ¿ American Geophysical Union 1994