Ethical theories typically run on moral principles : Kant's 'Always treat humanity, whether in your own person or in that of another, as an end and never merely as a means', say, or classical utilitarianism's 'Promote the greatest happiness of the greatest number'. Marx subscribed to moral principles : the formula for socialist justice is 'to each according to their work' and for communist justice, 'from each according to their ability, to each according to their needs'.
Ethical particularism, associated especially with the work of Jonathan Dancy, denies that moral judgements or moral reasoning need make any reference to moral principles whether these are conceived as (a) exceptionless rules or (b) default principles (considerations that apply, all else equal, or rules of thumb as one might say). It is against moral principles in sense (a) that ethical particularism is distinctively set.
One of the keystones of Dancy's position - taking Dancy as representative - is that we cannot properly subscribe in our moral judgements or moral reasoning to a moral consideration that automatically has force or has the same force in different situations. There is a stress on and recognition of the unique in Dancy's work. Take a consideration such as that one can cause pleasure. In one situation, causing pleasure may be a morally relevant and reason-giving consideration : it is good to give pleasure to A. In another situation, causing pleasure may be a morally relevant but repellent consideration : it is not good to give B pleasure by handing over an animal for B to delight in torturing. In yet another situation, such as that of whether to tell the truth, any pleasure involved may be totally irrelevant.
In sum, moral judgements and moral reasoning do not have to be, and in cannot adequately be, based on moral principles, exceptionless rules such as those defined by Kant or classical utilitarianism, or by Marx, or anyone else.
Very difficult issues are involved here; and Dancy's views have evolved. But I have sketched ethical particularism to illustrate an ethical theory that does not depend on moral principles. Dancy's views can be found in J. Dancy, 'Ethics Without Principles', Oxford, 2004. If you have any problems with this answer, just post a comment and I'll do my best to reply.