Spectral measurements of solar ultraviolet (UV) radiation received at the ground at Lauder, New Zealand (45¿S) during 1990 are used in conjunction with ozone total column measurment to investigate the relative importance to erythemally active UV radiation of variations in solar zenith angle, ozone, and cloud cover. At this site solar zenith angle variations are the dominant factor, but clouds frequently attenuate the clear sky irradiances by more than 50%. Ozone reductions of 1% typically cause an increase in erythemally active UV irradiance of 1.25¿0.20%. ¿American Geophysical Union 1991