I don't believe the question is necessarily a philosophical one, though the immediate borgeois temptation is to treat it as such. If Marx had his way, we wouldn't treat it this way, since philosophy is as idealist and mystificatory as religion.
Still, don't we usually think of the Marxist tradition(s) as largely a philosophical ones, and largely academic and bourgeois at that?
Most philosophy aside, I believe where people will divide over the issue is this belief: if you believe science is limited to the hypothetico-deductive method and controlled experimentation, then you will think that there is NO way Marxism is a science.
On the other hand, if you want to hold out more for science than the hypothetico-deductive method, then you can include the possibility that Marxist social sciences are scientific.
I suggest readings in, among others, Comte, Spencer, Marx, Engels (who, by the way, proclaimed the Marxian dialectic to be the science of human nature and society), Dilthey, Brentano, Husserl and Vygotsky. But I'm not going to attempt to recapitulate their thoughts on an e-mail list.
Here are some links to get you started:
http://www.marxists.org/reference/subject/philosophy/works/
http://www.marxists.org/reference/subject/philosophy/works/ge/dilthey1.htm
http://www.marxists.org/reference/subject/philosophy/works/ge/husserl.htm
http://www.marxists.org/archive/vygotsky/works/crisis/psycri01.htm#p100
My last word on the matter (not goodbye): I do not wish to limit human knowledge or even science to the hypothetico-deductive method. In as much as the social sciences are out of their philosophical stages, I can say I believe that Marxism is potentially scientific. Since Marxist oppositionism within bourgeois institutions (such as the university, but also the company and government) has given us so much useful critique of establishment knowledge, I would say, yes, I believe Marxism is scientific.
That is my final synthesis on the matter. Thank you for letting me get that out of my system.
Charles Jannuzi