Following marrow transplantation, the clinical course of the patients is invariably complicated by severe infections due to neutropenia and immuno-incompetence. With a view to explore means of alleviating these complications, Bacillus Calmette-Guerin (BCG), methanol extraction residue of BCG (MER), Corynebacterium parvum (C. parvum) and Pyran were examined for their myelopoietic activity on human marrow. Light density (less than or equal to 1.077 g/ml) unfractionated (UF) cells (2 X 10(6)/ml and 2ml/dish), and adherent (Ad) and non-adherent (N-Ad) cells alone (derived from 4 X 10(6) UF cells per dish) were incubated with and without the above agents. The conditioned media (CM) were harvested at 24, 72, 96, and 168 hours and colony stimulating activity (CSA) assayed against light density non-adherent human marrow cells using double layer agar-culture system. CSA was maximum at 72 and 96 hours followed by an invariable decline at 168 hours' incubation. The optimal concentrations releasing maximum CSA varied for each agent. CM prepared with higher concentrations were less active. CM prepared in the presence of Pyran had no CSA. Comparisons of the maximally released CSA by the optimal concentrations of these agents revealed BCG and MER to be the most active. The CSA elaborated by UF cells was more than the total combined activity from separately incubated Ad and N-Ad cells. Re-addition studies revealed that almost full CSA could be recovered when Ad and N-Ad cells were incubated together at a ratio of 1:3. Different immunoadjuvants have a differing capacity to elaborate CSA from human marrow cells and each has an optimal concentration for maximum CSA release. This may require a cell-cell and/or humoral interaction(s) between Ad and N-Ad cells.