The object of this study was to develop a method for the adjustment of test-day milk, fat and protein yield for age at calving, stage of lactation, and month of calving. Test-day records of 14 275 purebred Dutch Friesians were analysed by generalized least squares, (GLS). Functions of days in lactation and age in months were fitted to the age differences for different stages in lactation. A close fit to the GLS means was obtained for both milk fat and protein yield. From these functions, adjustment factors (CFagei) were computed depending on the cow's age in months and number of days in lactation. A multiplicative adjustment for age and stage of lactation by CFage~ was compared with an adjustment using factors derived from the constant estimates (CFag~iI). Comparison was on the F-variable and constant estimates for age after adjustment. When CF~get was used instead of CFagem lower F-values were obtained and the constants showed a more regular pattern. Additive adjustment factors for month of calving were derived from the GLS constant estimates. After correction, month of calving was only significant for protein yield. The adjustment of test-day yields for age at calving and stage of lactation was improved by using mathematical functions.