Health status and immunocompetence have been proposed as important fac
tors affecting individual variation in the attainment of breeding cond
ition in birds. We studied individual variation in serological variabl
es indicating health status (blood sedimentation rate, haematocrit, 'b
uffy coat' layer. proportions of different types of leucocytes) in two
groups of breeding chinstrap penguins Pygoscelis antarctica with bree
ding dates 9 days apart. We sampled these individuals shortly after ha
tching of their young and at the end of the chick-raising period. A gr
oup of failed breeders was also sampled. Birds of both sexes were incl
uded. We also measured the T-cell-mediated immune response as indicate
d by an in vivo hypersensitivity response to an intradermal injection
of a mitogen (phytohaemagglutinin) in early and late breeders. Sex had
no significant effect on most variables. Late breeders had poorer hea
lth (more leucocytes, especially heterophils and lymphocytes) and a lo
wer T-cell-mediated immune response than early breeders. Failed breede
rs were more similar to late than to early breeders. Early breeders su
ffered a decline in health status throughout the chick-raising period.
The impact of pathogens on variation in life history traits in avian
populations may be important even in extreme Antarctic environments.