Worthington Biochemical FAQ

The Neonatal Cardiomyocyte Isolation System is specifically packaged and use tested on neonatal rats. 

A few researchers have used the system on neonatal mice, with some modifications. I have copied some references below for neonatal mice. It seems neonatal mouse hearts may require higher enzyme concentrations and/or longer digestion times than the rats. Increasing the collagenase digestion time to 45 min is a good place to start your optimization. Also one researcher has informed me he increased the trypsin concentration to 100 ug/ml by cutting final volumes in half.

http://ajpcell.physiology.org/cgi/content/full/277/6/C1277#SEC1
http://www.jbc.org/cgi/content/full/277/50/48282
http://www.pnas.org/cgi/content/full/95/16/9669#MaterialsMethods

The protocol for the NCIS kit can viewed from the following link-
http://www.worthington-biochem.com/NCIS/default.html