Observations of 5 production traits on 4 000 individuals in the Dohne Merino Development Programme were used to estimate correction factors for the influence of age of dam, type of birth (single or twin) and sex of lamb on 5 traits. Estimates were based on a mathematical model fitted to the data and analysed by least-squares analysis of variance for data with unequal subclass numbers. All 3 non-genetic factors had a significant (P ≤0,01) influence on body mass at birth, 100 days, and at 12 and 18 months of age. Only type of birth and sex of lamb had a significant influence (P ≤0,01) on greasy fleece mass at 18 months of age. Age of dam and type of birth had a diminishing influence on body mass traits as the lambs grew older. The influence of sex of lamb on body mass increased with increase in age. The magnitude of the correction factors for the influence of the 3 non-genetic factors on greasy fleece mass at l8 months of age was relatively smaller than for their influence on body mass traits. For more accurate selection it is recommended that measurements on body mass and wool production, in growing Dohne Merino sheep, should firstly be corrected for differences in type of birth and then for differences in age of dam. However, data utilised for the estimation of genetic parameters should be corrected also for the influence of sex of lamb on the production traits studied