OBJECTIVE To examine systemic immunity in kittens, including transfer of maternal immunoglobulins from the queen to kittens, and subsequent decay of passively obtained immunoglobulins. METHODS 6 healthy queens and their 46 kittens. METHODS Immunoglobulin concentrations were measured in serum, colostrum, and milk of queens and in their kittens' sera. Decay rate constants and half-lives of maternally derived immunoglobulins were determined. To determine intestinal absorption, foreign IgG was given to kittens at 6- to 8-hour intervals after birth, and bovine IgM was given to kittens at birth. RESULTS Immunoglobulin concentrations of milk and colostrum did not differ significantly after removal of milk fat. Mean IgG concentration was higher in colostrum/ milk, whereas mean IgA and IgM concentrations were lower than those in the queens' serum. No IgG or IgA was detected in any of the precolostral serum samples obtained from kittens. Small amounts of IgM were present in the sera from 5 kittens at birth. Transferred IgG and IgA decreased rapidly with half-lives of 4.4 +/- 3.57 and 1.93 +/- 1.94 days, respectively. Serum IgM concentration increased irregularly during the first week of life, followed by a steady increase. Foreign IgG given up to 12 hours after birth was detected in kittens' serum, whereas IgG given at or after 16 hours was not found in any kitten's serum. CONCLUSIONS Milk and colostral immunoglobulin concentrations did not differ significantly. The half-lives of maternally derived IgG and IgA in kittens were shorter than those reported in dogs. IgG given at or after 16 hours of life was not absorbed by neonatal kittens. CONCLUSIONS Queen's milk obtained anytime during lactation may be used as a replacement for colostrum as a source of antibodies for neonatal kittens. Kittens at risk for neonatal isoerythrolysis must only be removed from the queens during the first day of life.