Different molecular species of interleukin 1 (IL 1) were examined for the spectrum of responses elicited in human endothelial cells (HEC), including synthesis of prostacyclin (PGI2), tissue-type procoagulant activity (PCA), platelet activating factor (PAF), and plasminogen activator inhibitor (PA-I). The IL 1 preparations utilized for the present study included a natural, partially purified IL 1, a preparation purified to homogeneity with extensive homology with the derived aminoacid IL 1 beta (pI7) sequence denominated “22K factor,” murine recombinant IL 1 alpha, human recombinant IL 1 alpha (pI5) and beta (pI7). Natural, partially purified IL 1, a mixture of alpha and beta species, induced the entire spectrum of responses in HEC. Production of PA-I was elicited by all forms of IL 1 tested. PGI2 and PCA were elicited by “22K factor” and by human recombinant IL 1 beta and alpha but not by murine recombinant IL 1 alpha. PAF synthesis was stimulated by murine and human recombinant IL 1 alpha but not by human recombinant IL 1 beta and 22K factor. Thus the available different molecular forms of IL 1 elicit largely but not completely overlapping patterns of responses in HEC. The IL 1 pathway of regulation of HEC functions might provide a basis for novel strategies in therapeutically oriented research on vessel wall disorders.