DMSO differentiated U937 cells responded to 10⁻⁶ M LTD₄, LTB₄ and FMLP with an increase in both InsP formation and [Ca²⁺]_i. FMLP caused a greater rise in InsPs than either LTD₄ or LTB₄, which were equivalent. LTD₄, however, caused a greater increase in [Ca²⁺]_i than LTB₄ (4-fold) or FMLP. The FMLP [Ca²⁺]_i and InsP responses were abolished by pertussis toxin (100 ng/ml for 4 h) but were unaffected by PMA (10⁻⁷ M for 3 min). In contrast, the LTD₄ [Ca²⁺]_i and InsP responses were reduced by only 50% by pertussis toxin, whilst PMA reduced the [Ca²⁺]_i and InsP responses to LTD₄ by 75 and 30%, respectively. These results suggest that mechanisms additional to InsP formation exist for mediating LTD₄ evoked increases in [Ca²⁺]_i.