I would kindly advise against such definitions since is a far more convenient and useful alternative in enumitem
:
\documentclass{article}
\usepackage{enumitem}% http://ctan.org/pkg/enumitem
\begin{document}
\begin{enumerate}[label=\arabic*.,ref=\arabic*]
\item Some item
\begin{enumerate}[label*=\arabic*.,ref=\theenumi.\arabic*]
\item Another item \label{myitem}
\item Yet another item
\end{enumerate}
\end{enumerate}
See Item~\ref{myitem}.
\end{document}
Note that different label
and ref
key-values are supplied, since the former includes a trailing period .
that you don't want to include in the latter. Using label*
inherits the label
from the higher level in the nested environment.