Assay of prolactin after freezing cow's milk

Concentrations of prolactin were similar in aliquots of the same milk sample stored for 2 days at 4 C or --23 C but averaged lower if prepared for assay at 30 C than if prepared at 40 or 50 C. Average deviations in prolactin between duplicate measurements relative to respective treatment averages were lowest generally when technique of mixing included vortexing for 5 s immediately prior to pipetting volumes of milk for assay. The average deviation of differences in prolactin among duplicate measurements relative to average concentrations of prolactin were about the same for colostrum, milk, and blood plasma (11, 17, and 14%) stored frozen. Prolactin can be measured reliably in frozen cow's milk provided samples are warmed to 40 C to 50 C and mixed thoroughly when prepared for assay.