Pseudothrombocytopenia may have any of a number of causes, one of which is agglutination in vitro. This phenomenon was found in samples of blood from six patients. A serum factor responsible for the agglutination was demonstrated. The factor was dependent upon the presence of EDTA and was more active at room temperature than at 37 C. It could be identified as an IgM immunoglobulin in four cases. In the other two cases definite characterization was not possible, but there was some evidence in favor of an IgM factor. All six patients had elevated serum IgM levels, but they had different and unrelated clinical disorders.

A group of 40 patients with idiopathic thrombocytopenic purpura (ITP) was examined. Typing for HLA A and B antigens was performed. The frequencies of 24 HLA antigens in this group were compared with those of a group of 13 patients with drug-induced thrombocytopenia and two large groups of healthy controls. Differences of statistical significance between these groups were not found. Thus, associations of ITP and particular HLA antigens were not observed. The associations reported by others could not be confirmed.

A bacterial infection was considered to be responsible for provoking or maintaining episodes of thrombocytopenia in 5 patients with chronic idiopathic thrombocytopenic purpura (ITP). In 3 patients the course of the disease was continuous and in 2 of them remission was attained after eradication of the infection. In the other 2 patients the course was intermittent and a number of relapses was provoked by the infection. The possible causal connection of chronic ITP and bacterial infections is briefly discussed.

Monoclonal immunoglobulins with affinity for platelets were detected in the blood of seven patients. Two of these had thrombocytopenia and non-Hodgkin's lymphoma (NHL). One patient had thrombocytopenia and possibly incipient NHL. The other four patients had pseudothrombocytopenia at the time of diagnosis but one of them developed NHL six years later. It is suggested that these monoclonal immunoglobulins may in some cases be associated with malignant lymphoma and that subjects presenting with these immunoglobulins should have a long term follow-up in order to elucidate the question whether or not lymphoma will develop.

A case of Werlhof's disease (immune thrombocytopenia) associated with myasthenia gravis is described. The two disorders developed within a few months of each other. The immunological and practical aspects of the association are discussed.